LCOV - code coverage report
Current view: top level - kernel/locking - rwsem.c (source / functions) Hit Total Coverage
Test: landlock.info Lines: 408 508 80.3 %
Date: 2021-04-22 12:43:58 Functions: 34 44 77.3 %

          Line data    Source code
       1             : // SPDX-License-Identifier: GPL-2.0
       2             : /* kernel/rwsem.c: R/W semaphores, public implementation
       3             :  *
       4             :  * Written by David Howells (dhowells@redhat.com).
       5             :  * Derived from asm-i386/semaphore.h
       6             :  *
       7             :  * Writer lock-stealing by Alex Shi <alex.shi@intel.com>
       8             :  * and Michel Lespinasse <walken@google.com>
       9             :  *
      10             :  * Optimistic spinning by Tim Chen <tim.c.chen@intel.com>
      11             :  * and Davidlohr Bueso <davidlohr@hp.com>. Based on mutexes.
      12             :  *
      13             :  * Rwsem count bit fields re-definition and rwsem rearchitecture by
      14             :  * Waiman Long <longman@redhat.com> and
      15             :  * Peter Zijlstra <peterz@infradead.org>.
      16             :  */
      17             : 
      18             : #include <linux/types.h>
      19             : #include <linux/kernel.h>
      20             : #include <linux/sched.h>
      21             : #include <linux/sched/rt.h>
      22             : #include <linux/sched/task.h>
      23             : #include <linux/sched/debug.h>
      24             : #include <linux/sched/wake_q.h>
      25             : #include <linux/sched/signal.h>
      26             : #include <linux/sched/clock.h>
      27             : #include <linux/export.h>
      28             : #include <linux/rwsem.h>
      29             : #include <linux/atomic.h>
      30             : 
      31             : #include "lock_events.h"
      32             : 
      33             : /*
      34             :  * The least significant 2 bits of the owner value has the following
      35             :  * meanings when set.
      36             :  *  - Bit 0: RWSEM_READER_OWNED - The rwsem is owned by readers
      37             :  *  - Bit 1: RWSEM_NONSPINNABLE - Cannot spin on a reader-owned lock
      38             :  *
      39             :  * When the rwsem is reader-owned and a spinning writer has timed out,
      40             :  * the nonspinnable bit will be set to disable optimistic spinning.
      41             : 
      42             :  * When a writer acquires a rwsem, it puts its task_struct pointer
      43             :  * into the owner field. It is cleared after an unlock.
      44             :  *
      45             :  * When a reader acquires a rwsem, it will also puts its task_struct
      46             :  * pointer into the owner field with the RWSEM_READER_OWNED bit set.
      47             :  * On unlock, the owner field will largely be left untouched. So
      48             :  * for a free or reader-owned rwsem, the owner value may contain
      49             :  * information about the last reader that acquires the rwsem.
      50             :  *
      51             :  * That information may be helpful in debugging cases where the system
      52             :  * seems to hang on a reader owned rwsem especially if only one reader
      53             :  * is involved. Ideally we would like to track all the readers that own
      54             :  * a rwsem, but the overhead is simply too big.
      55             :  *
      56             :  * A fast path reader optimistic lock stealing is supported when the rwsem
      57             :  * is previously owned by a writer and the following conditions are met:
      58             :  *  - OSQ is empty
      59             :  *  - rwsem is not currently writer owned
      60             :  *  - the handoff isn't set.
      61             :  */
      62             : #define RWSEM_READER_OWNED      (1UL << 0)
      63             : #define RWSEM_NONSPINNABLE      (1UL << 1)
      64             : #define RWSEM_OWNER_FLAGS_MASK  (RWSEM_READER_OWNED | RWSEM_NONSPINNABLE)
      65             : 
      66             : #ifdef CONFIG_DEBUG_RWSEMS
      67             : # define DEBUG_RWSEMS_WARN_ON(c, sem)   do {                    \
      68             :         if (!debug_locks_silent &&                              \
      69             :             WARN_ONCE(c, "DEBUG_RWSEMS_WARN_ON(%s): count = 0x%lx, magic = 0x%lx, owner = 0x%lx, curr 0x%lx, list %sempty\n",\
      70             :                 #c, atomic_long_read(&(sem)->count),             \
      71             :                 (unsigned long) sem->magic,                  \
      72             :                 atomic_long_read(&(sem)->owner), (long)current,  \
      73             :                 list_empty(&(sem)->wait_list) ? "" : "not "))        \
      74             :                         debug_locks_off();                      \
      75             :         } while (0)
      76             : #else
      77             : # define DEBUG_RWSEMS_WARN_ON(c, sem)
      78             : #endif
      79             : 
      80             : /*
      81             :  * On 64-bit architectures, the bit definitions of the count are:
      82             :  *
      83             :  * Bit  0    - writer locked bit
      84             :  * Bit  1    - waiters present bit
      85             :  * Bit  2    - lock handoff bit
      86             :  * Bits 3-7  - reserved
      87             :  * Bits 8-62 - 55-bit reader count
      88             :  * Bit  63   - read fail bit
      89             :  *
      90             :  * On 32-bit architectures, the bit definitions of the count are:
      91             :  *
      92             :  * Bit  0    - writer locked bit
      93             :  * Bit  1    - waiters present bit
      94             :  * Bit  2    - lock handoff bit
      95             :  * Bits 3-7  - reserved
      96             :  * Bits 8-30 - 23-bit reader count
      97             :  * Bit  31   - read fail bit
      98             :  *
      99             :  * It is not likely that the most significant bit (read fail bit) will ever
     100             :  * be set. This guard bit is still checked anyway in the down_read() fastpath
     101             :  * just in case we need to use up more of the reader bits for other purpose
     102             :  * in the future.
     103             :  *
     104             :  * atomic_long_fetch_add() is used to obtain reader lock, whereas
     105             :  * atomic_long_cmpxchg() will be used to obtain writer lock.
     106             :  *
     107             :  * There are three places where the lock handoff bit may be set or cleared.
     108             :  * 1) rwsem_mark_wake() for readers.
     109             :  * 2) rwsem_try_write_lock() for writers.
     110             :  * 3) Error path of rwsem_down_write_slowpath().
     111             :  *
     112             :  * For all the above cases, wait_lock will be held. A writer must also
     113             :  * be the first one in the wait_list to be eligible for setting the handoff
     114             :  * bit. So concurrent setting/clearing of handoff bit is not possible.
     115             :  */
     116             : #define RWSEM_WRITER_LOCKED     (1UL << 0)
     117             : #define RWSEM_FLAG_WAITERS      (1UL << 1)
     118             : #define RWSEM_FLAG_HANDOFF      (1UL << 2)
     119             : #define RWSEM_FLAG_READFAIL     (1UL << (BITS_PER_LONG - 1))
     120             : 
     121             : #define RWSEM_READER_SHIFT      8
     122             : #define RWSEM_READER_BIAS       (1UL << RWSEM_READER_SHIFT)
     123             : #define RWSEM_READER_MASK       (~(RWSEM_READER_BIAS - 1))
     124             : #define RWSEM_WRITER_MASK       RWSEM_WRITER_LOCKED
     125             : #define RWSEM_LOCK_MASK         (RWSEM_WRITER_MASK|RWSEM_READER_MASK)
     126             : #define RWSEM_READ_FAILED_MASK  (RWSEM_WRITER_MASK|RWSEM_FLAG_WAITERS|\
     127             :                                  RWSEM_FLAG_HANDOFF|RWSEM_FLAG_READFAIL)
     128             : 
     129             : /*
     130             :  * All writes to owner are protected by WRITE_ONCE() to make sure that
     131             :  * store tearing can't happen as optimistic spinners may read and use
     132             :  * the owner value concurrently without lock. Read from owner, however,
     133             :  * may not need READ_ONCE() as long as the pointer value is only used
     134             :  * for comparison and isn't being dereferenced.
     135             :  */
     136      328859 : static inline void rwsem_set_owner(struct rw_semaphore *sem)
     137             : {
     138      657679 :         atomic_long_set(&sem->owner, (long)current);
     139             : }
     140             : 
     141      327639 : static inline void rwsem_clear_owner(struct rw_semaphore *sem)
     142             : {
     143      655299 :         atomic_long_set(&sem->owner, 0);
     144             : }
     145             : 
     146             : /*
     147             :  * Test the flags in the owner field.
     148             :  */
     149      181648 : static inline bool rwsem_test_oflags(struct rw_semaphore *sem, long flags)
     150             : {
     151           0 :         return atomic_long_read(&sem->owner) & flags;
     152             : }
     153             : 
     154             : /*
     155             :  * The task_struct pointer of the last owning reader will be left in
     156             :  * the owner field.
     157             :  *
     158             :  * Note that the owner value just indicates the task has owned the rwsem
     159             :  * previously, it may not be the real owner or one of the real owners
     160             :  * anymore when that field is examined, so take it with a grain of salt.
     161             :  *
     162             :  * The reader non-spinnable bit is preserved.
     163             :  */
     164      181428 : static inline void __rwsem_set_reader_owned(struct rw_semaphore *sem,
     165             :                                             struct task_struct *owner)
     166             : {
     167      362845 :         unsigned long val = (unsigned long)owner | RWSEM_READER_OWNED |
     168      181428 :                 (atomic_long_read(&sem->owner) & RWSEM_NONSPINNABLE);
     169             : 
     170      181417 :         atomic_long_set(&sem->owner, val);
     171      181427 : }
     172             : 
     173      181388 : static inline void rwsem_set_reader_owned(struct rw_semaphore *sem)
     174             : {
     175      181388 :         __rwsem_set_reader_owned(sem, current);
     176             : }
     177             : 
     178             : /*
     179             :  * Return true if the rwsem is owned by a reader.
     180             :  */
     181      181556 : static inline bool is_rwsem_reader_owned(struct rw_semaphore *sem)
     182             : {
     183             : #ifdef CONFIG_DEBUG_RWSEMS
     184             :         /*
     185             :          * Check the count to see if it is write-locked.
     186             :          */
     187      181556 :         long count = atomic_long_read(&sem->count);
     188             : 
     189      181540 :         if (count & RWSEM_WRITER_MASK)
     190             :                 return false;
     191             : #endif
     192      181545 :         return rwsem_test_oflags(sem, RWSEM_READER_OWNED);
     193             : }
     194             : 
     195             : #ifdef CONFIG_DEBUG_RWSEMS
     196             : /*
     197             :  * With CONFIG_DEBUG_RWSEMS configured, it will make sure that if there
     198             :  * is a task pointer in owner of a reader-owned rwsem, it will be the
     199             :  * real owner or one of the real owners. The only exception is when the
     200             :  * unlock is done by up_read_non_owner().
     201             :  */
     202      181442 : static inline void rwsem_clear_reader_owned(struct rw_semaphore *sem)
     203             : {
     204      181442 :         unsigned long val = atomic_long_read(&sem->owner);
     205             : 
     206      181431 :         while ((val & ~RWSEM_OWNER_FLAGS_MASK) == (unsigned long)current) {
     207      181348 :                 if (atomic_long_try_cmpxchg(&sem->owner, &val,
     208      181323 :                                             val & RWSEM_OWNER_FLAGS_MASK))
     209      181348 :                         return;
     210             :         }
     211             : }
     212             : #else
     213             : static inline void rwsem_clear_reader_owned(struct rw_semaphore *sem)
     214             : {
     215             : }
     216             : #endif
     217             : 
     218             : /*
     219             :  * Set the RWSEM_NONSPINNABLE bits if the RWSEM_READER_OWNED flag
     220             :  * remains set. Otherwise, the operation will be aborted.
     221             :  */
     222          32 : static inline void rwsem_set_nonspinnable(struct rw_semaphore *sem)
     223             : {
     224          32 :         unsigned long owner = atomic_long_read(&sem->owner);
     225             : 
     226          32 :         do {
     227          32 :                 if (!(owner & RWSEM_READER_OWNED))
     228             :                         break;
     229          32 :                 if (owner & RWSEM_NONSPINNABLE)
     230             :                         break;
     231          64 :         } while (!atomic_long_try_cmpxchg(&sem->owner, &owner,
     232          64 :                                           owner | RWSEM_NONSPINNABLE));
     233          32 : }
     234             : 
     235       35892 : static inline bool rwsem_read_trylock(struct rw_semaphore *sem, long *cntp)
     236             : {
     237       35892 :         *cntp = atomic_long_add_return_acquire(RWSEM_READER_BIAS, &sem->count);
     238             : 
     239       35894 :         if (WARN_ON_ONCE(*cntp < 0))
     240           0 :                 rwsem_set_nonspinnable(sem);
     241             : 
     242       35894 :         if (!(*cntp & RWSEM_READ_FAILED_MASK)) {
     243       35783 :                 rwsem_set_reader_owned(sem);
     244       35783 :                 return true;
     245             :         }
     246             : 
     247             :         return false;
     248             : }
     249             : 
     250      328804 : static inline bool rwsem_write_trylock(struct rw_semaphore *sem)
     251             : {
     252      328804 :         long tmp = RWSEM_UNLOCKED_VALUE;
     253             : 
     254      657663 :         if (atomic_long_try_cmpxchg_acquire(&sem->count, &tmp, RWSEM_WRITER_LOCKED)) {
     255      328389 :                 rwsem_set_owner(sem);
     256      328350 :                 return true;
     257             :         }
     258             : 
     259             :         return false;
     260             : }
     261             : 
     262             : /*
     263             :  * Return just the real task structure pointer of the owner
     264             :  */
     265      328796 : static inline struct task_struct *rwsem_owner(struct rw_semaphore *sem)
     266             : {
     267      657592 :         return (struct task_struct *)
     268      657592 :                 (atomic_long_read(&sem->owner) & ~RWSEM_OWNER_FLAGS_MASK);
     269             : }
     270             : 
     271             : /*
     272             :  * Return the real task structure pointer of the owner and the embedded
     273             :  * flags in the owner. pflags must be non-NULL.
     274             :  */
     275             : static inline struct task_struct *
     276      225424 : rwsem_owner_flags(struct rw_semaphore *sem, unsigned long *pflags)
     277             : {
     278      450848 :         unsigned long owner = atomic_long_read(&sem->owner);
     279             : 
     280      225424 :         *pflags = owner & RWSEM_OWNER_FLAGS_MASK;
     281      225424 :         return (struct task_struct *)(owner & ~RWSEM_OWNER_FLAGS_MASK);
     282             : }
     283             : 
     284             : /*
     285             :  * Guide to the rw_semaphore's count field.
     286             :  *
     287             :  * When the RWSEM_WRITER_LOCKED bit in count is set, the lock is owned
     288             :  * by a writer.
     289             :  *
     290             :  * The lock is owned by readers when
     291             :  * (1) the RWSEM_WRITER_LOCKED isn't set in count,
     292             :  * (2) some of the reader bits are set in count, and
     293             :  * (3) the owner field has RWSEM_READ_OWNED bit set.
     294             :  *
     295             :  * Having some reader bits set is not enough to guarantee a readers owned
     296             :  * lock as the readers may be in the process of backing out from the count
     297             :  * and a writer has just released the lock. So another writer may steal
     298             :  * the lock immediately after that.
     299             :  */
     300             : 
     301             : /*
     302             :  * Initialize an rwsem:
     303             :  */
     304       58145 : void __init_rwsem(struct rw_semaphore *sem, const char *name,
     305             :                   struct lock_class_key *key)
     306             : {
     307             : #ifdef CONFIG_DEBUG_LOCK_ALLOC
     308             :         /*
     309             :          * Make sure we are not reinitializing a held semaphore:
     310             :          */
     311       58145 :         debug_check_no_locks_freed((void *)sem, sizeof(*sem));
     312       58145 :         lockdep_init_map_wait(&sem->dep_map, name, key, 0, LD_WAIT_SLEEP);
     313             : #endif
     314             : #ifdef CONFIG_DEBUG_RWSEMS
     315       58145 :         sem->magic = sem;
     316             : #endif
     317       58145 :         atomic_long_set(&sem->count, RWSEM_UNLOCKED_VALUE);
     318       58145 :         raw_spin_lock_init(&sem->wait_lock);
     319       58146 :         INIT_LIST_HEAD(&sem->wait_list);
     320       58146 :         atomic_long_set(&sem->owner, 0L);
     321             : #ifdef CONFIG_RWSEM_SPIN_ON_OWNER
     322       58146 :         osq_lock_init(&sem->osq);
     323             : #endif
     324       58146 : }
     325             : EXPORT_SYMBOL(__init_rwsem);
     326             : 
     327             : enum rwsem_waiter_type {
     328             :         RWSEM_WAITING_FOR_WRITE,
     329             :         RWSEM_WAITING_FOR_READ
     330             : };
     331             : 
     332             : struct rwsem_waiter {
     333             :         struct list_head list;
     334             :         struct task_struct *task;
     335             :         enum rwsem_waiter_type type;
     336             :         unsigned long timeout;
     337             : };
     338             : #define rwsem_first_waiter(sem) \
     339             :         list_first_entry(&sem->wait_list, struct rwsem_waiter, list)
     340             : 
     341             : enum rwsem_wake_type {
     342             :         RWSEM_WAKE_ANY,         /* Wake whatever's at head of wait list */
     343             :         RWSEM_WAKE_READERS,     /* Wake readers only */
     344             :         RWSEM_WAKE_READ_OWNED   /* Waker thread holds the read lock */
     345             : };
     346             : 
     347             : enum writer_wait_state {
     348             :         WRITER_NOT_FIRST,       /* Writer is not first in wait list */
     349             :         WRITER_FIRST,           /* Writer is first in wait list     */
     350             :         WRITER_HANDOFF          /* Writer is first & handoff needed */
     351             : };
     352             : 
     353             : /*
     354             :  * The typical HZ value is either 250 or 1000. So set the minimum waiting
     355             :  * time to at least 4ms or 1 jiffy (if it is higher than 4ms) in the wait
     356             :  * queue before initiating the handoff protocol.
     357             :  */
     358             : #define RWSEM_WAIT_TIMEOUT      DIV_ROUND_UP(HZ, 250)
     359             : 
     360             : /*
     361             :  * Magic number to batch-wakeup waiting readers, even when writers are
     362             :  * also present in the queue. This both limits the amount of work the
     363             :  * waking thread must do and also prevents any potential counter overflow,
     364             :  * however unlikely.
     365             :  */
     366             : #define MAX_READERS_WAKEUP      0x100
     367             : 
     368             : /*
     369             :  * handle the lock release when processes blocked on it that can now run
     370             :  * - if we come here from up_xxxx(), then the RWSEM_FLAG_WAITERS bit must
     371             :  *   have been set.
     372             :  * - there must be someone on the queue
     373             :  * - the wait_lock must be held by the caller
     374             :  * - tasks are marked for wakeup, the caller must later invoke wake_up_q()
     375             :  *   to actually wakeup the blocked task(s) and drop the reference count,
     376             :  *   preferably when the wait_lock is released
     377             :  * - woken process blocks are discarded from the list after having task zeroed
     378             :  * - writers are only marked woken if downgrading is false
     379             :  */
     380         287 : static void rwsem_mark_wake(struct rw_semaphore *sem,
     381             :                             enum rwsem_wake_type wake_type,
     382             :                             struct wake_q_head *wake_q)
     383             : {
     384         287 :         struct rwsem_waiter *waiter, *tmp;
     385         287 :         long oldcount, woken = 0, adjustment = 0;
     386         287 :         struct list_head wlist;
     387             : 
     388         574 :         lockdep_assert_held(&sem->wait_lock);
     389             : 
     390             :         /*
     391             :          * Take a peek at the queue head waiter such that we can determine
     392             :          * the wakeup(s) to perform.
     393             :          */
     394         287 :         waiter = rwsem_first_waiter(sem);
     395             : 
     396         287 :         if (waiter->type == RWSEM_WAITING_FOR_WRITE) {
     397         242 :                 if (wake_type == RWSEM_WAKE_ANY) {
     398             :                         /*
     399             :                          * Mark writer at the front of the queue for wakeup.
     400             :                          * Until the task is actually later awoken later by
     401             :                          * the caller, other writers are able to steal it.
     402             :                          * Readers, on the other hand, will block as they
     403             :                          * will notice the queued writer.
     404             :                          */
     405         192 :                         wake_q_add(wake_q, waiter->task);
     406         242 :                         lockevent_inc(rwsem_wake_writer);
     407             :                 }
     408             : 
     409         243 :                 return;
     410             :         }
     411             : 
     412             :         /*
     413             :          * No reader wakeup if there are too many of them already.
     414             :          */
     415          45 :         if (unlikely(atomic_long_read(&sem->count) < 0))
     416             :                 return;
     417             : 
     418             :         /*
     419             :          * Writers might steal the lock before we grant it to the next reader.
     420             :          * We prefer to do the first reader grant before counting readers
     421             :          * so we can bail out early if a writer stole the lock.
     422             :          */
     423          45 :         if (wake_type != RWSEM_WAKE_READ_OWNED) {
     424          45 :                 struct task_struct *owner;
     425             : 
     426          45 :                 adjustment = RWSEM_READER_BIAS;
     427          45 :                 oldcount = atomic_long_fetch_add(adjustment, &sem->count);
     428          45 :                 if (unlikely(oldcount & RWSEM_WRITER_MASK)) {
     429             :                         /*
     430             :                          * When we've been waiting "too" long (for writers
     431             :                          * to give up the lock), request a HANDOFF to
     432             :                          * force the issue.
     433             :                          */
     434           1 :                         if (!(oldcount & RWSEM_FLAG_HANDOFF) &&
     435           1 :                             time_after(jiffies, waiter->timeout)) {
     436           1 :                                 adjustment -= RWSEM_FLAG_HANDOFF;
     437           1 :                                 lockevent_inc(rwsem_rlock_handoff);
     438             :                         }
     439             : 
     440           1 :                         atomic_long_add(-adjustment, &sem->count);
     441           1 :                         return;
     442             :                 }
     443             :                 /*
     444             :                  * Set it to reader-owned to give spinners an early
     445             :                  * indication that readers now have the lock.
     446             :                  * The reader nonspinnable bit seen at slowpath entry of
     447             :                  * the reader is copied over.
     448             :                  */
     449          44 :                 owner = waiter->task;
     450          44 :                 __rwsem_set_reader_owned(sem, owner);
     451             :         }
     452             : 
     453             :         /*
     454             :          * Grant up to MAX_READERS_WAKEUP read locks to all the readers in the
     455             :          * queue. We know that the woken will be at least 1 as we accounted
     456             :          * for above. Note we increment the 'active part' of the count by the
     457             :          * number of readers before waking any processes up.
     458             :          *
     459             :          * This is an adaptation of the phase-fair R/W locks where at the
     460             :          * reader phase (first waiter is a reader), all readers are eligible
     461             :          * to acquire the lock at the same time irrespective of their order
     462             :          * in the queue. The writers acquire the lock according to their
     463             :          * order in the queue.
     464             :          *
     465             :          * We have to do wakeup in 2 passes to prevent the possibility that
     466             :          * the reader count may be decremented before it is incremented. It
     467             :          * is because the to-be-woken waiter may not have slept yet. So it
     468             :          * may see waiter->task got cleared, finish its critical section and
     469             :          * do an unlock before the reader count increment.
     470             :          *
     471             :          * 1) Collect the read-waiters in a separate list, count them and
     472             :          *    fully increment the reader count in rwsem.
     473             :          * 2) For each waiters in the new list, clear waiter->task and
     474             :          *    put them into wake_q to be woken up later.
     475             :          */
     476          44 :         INIT_LIST_HEAD(&wlist);
     477         125 :         list_for_each_entry_safe(waiter, tmp, &sem->wait_list, list) {
     478          81 :                 if (waiter->type == RWSEM_WAITING_FOR_WRITE)
     479           8 :                         continue;
     480             : 
     481          73 :                 woken++;
     482          73 :                 list_move_tail(&waiter->list, &wlist);
     483             : 
     484             :                 /*
     485             :                  * Limit # of readers that can be woken up per wakeup call.
     486             :                  */
     487          73 :                 if (woken >= MAX_READERS_WAKEUP)
     488             :                         break;
     489             :         }
     490             : 
     491          44 :         adjustment = woken * RWSEM_READER_BIAS - adjustment;
     492          44 :         lockevent_cond_inc(rwsem_wake_reader, woken);
     493          44 :         if (list_empty(&sem->wait_list)) {
     494             :                 /* hit end of list above */
     495          41 :                 adjustment -= RWSEM_FLAG_WAITERS;
     496             :         }
     497             : 
     498             :         /*
     499             :          * When we've woken a reader, we no longer need to force writers
     500             :          * to give up the lock and we can clear HANDOFF.
     501             :          */
     502          44 :         if (woken && (atomic_long_read(&sem->count) & RWSEM_FLAG_HANDOFF))
     503           1 :                 adjustment -= RWSEM_FLAG_HANDOFF;
     504             : 
     505          44 :         if (adjustment)
     506          44 :                 atomic_long_add(adjustment, &sem->count);
     507             : 
     508             :         /* 2nd pass */
     509         117 :         list_for_each_entry_safe(waiter, tmp, &wlist, list) {
     510          73 :                 struct task_struct *tsk;
     511             : 
     512          73 :                 tsk = waiter->task;
     513          73 :                 get_task_struct(tsk);
     514             : 
     515             :                 /*
     516             :                  * Ensure calling get_task_struct() before setting the reader
     517             :                  * waiter to nil such that rwsem_down_read_slowpath() cannot
     518             :                  * race with do_exit() by always holding a reference count
     519             :                  * to the task to wakeup.
     520             :                  */
     521          73 :                 smp_store_release(&waiter->task, NULL);
     522             :                 /*
     523             :                  * Ensure issuing the wakeup (either by us or someone else)
     524             :                  * after setting the reader waiter to nil.
     525             :                  */
     526          73 :                 wake_q_add_safe(wake_q, tsk);
     527             :         }
     528             : }
     529             : 
     530             : /*
     531             :  * This function must be called with the sem->wait_lock held to prevent
     532             :  * race conditions between checking the rwsem wait list and setting the
     533             :  * sem->count accordingly.
     534             :  *
     535             :  * If wstate is WRITER_HANDOFF, it will make sure that either the handoff
     536             :  * bit is set or the lock is acquired with handoff bit cleared.
     537             :  */
     538         170 : static inline bool rwsem_try_write_lock(struct rw_semaphore *sem,
     539             :                                         enum writer_wait_state wstate)
     540             : {
     541         170 :         long count, new;
     542             : 
     543         340 :         lockdep_assert_held(&sem->wait_lock);
     544             : 
     545         170 :         count = atomic_long_read(&sem->count);
     546         170 :         do {
     547         170 :                 bool has_handoff = !!(count & RWSEM_FLAG_HANDOFF);
     548             : 
     549         170 :                 if (has_handoff && wstate == WRITER_NOT_FIRST)
     550             :                         return false;
     551             : 
     552         166 :                 new = count;
     553             : 
     554         166 :                 if (count & RWSEM_LOCK_MASK) {
     555          77 :                         if (has_handoff || (wstate != WRITER_HANDOFF))
     556             :                                 return false;
     557             : 
     558           4 :                         new |= RWSEM_FLAG_HANDOFF;
     559             :                 } else {
     560          89 :                         new |= RWSEM_WRITER_LOCKED;
     561          89 :                         new &= ~RWSEM_FLAG_HANDOFF;
     562             : 
     563          89 :                         if (list_is_singular(&sem->wait_list))
     564          43 :                                 new &= ~RWSEM_FLAG_WAITERS;
     565             :                 }
     566         186 :         } while (!atomic_long_try_cmpxchg_acquire(&sem->count, &count, new));
     567             : 
     568             :         /*
     569             :          * We have either acquired the lock with handoff bit cleared or
     570             :          * set the handoff bit.
     571             :          */
     572          93 :         if (new & RWSEM_FLAG_HANDOFF)
     573             :                 return false;
     574             : 
     575          89 :         rwsem_set_owner(sem);
     576          89 :         return true;
     577             : }
     578             : 
     579             : #ifdef CONFIG_RWSEM_SPIN_ON_OWNER
     580             : /*
     581             :  * Try to acquire write lock before the writer has been put on wait queue.
     582             :  */
     583        2894 : static inline bool rwsem_try_write_lock_unqueued(struct rw_semaphore *sem)
     584             : {
     585        2894 :         long count = atomic_long_read(&sem->count);
     586             : 
     587        2894 :         while (!(count & (RWSEM_LOCK_MASK|RWSEM_FLAG_HANDOFF))) {
     588         381 :                 if (atomic_long_try_cmpxchg_acquire(&sem->count, &count,
     589         381 :                                         count | RWSEM_WRITER_LOCKED)) {
     590         381 :                         rwsem_set_owner(sem);
     591         381 :                         lockevent_inc(rwsem_opt_lock);
     592         381 :                         return true;
     593             :                 }
     594             :         }
     595             :         return false;
     596             : }
     597             : 
     598      222099 : static inline bool owner_on_cpu(struct task_struct *owner)
     599             : {
     600             :         /*
     601             :          * As lock holder preemption issue, we both skip spinning if
     602             :          * task is not on cpu or its cpu is preempted
     603             :          */
     604      222099 :         return owner->on_cpu && !vcpu_is_preempted(task_cpu(owner));
     605             : }
     606             : 
     607         470 : static inline bool rwsem_can_spin_on_owner(struct rw_semaphore *sem)
     608             : {
     609         470 :         struct task_struct *owner;
     610         470 :         unsigned long flags;
     611         470 :         bool ret = true;
     612             : 
     613         470 :         if (need_resched()) {
     614             :                 lockevent_inc(rwsem_opt_fail);
     615             :                 return false;
     616             :         }
     617             : 
     618         453 :         preempt_disable();
     619         453 :         rcu_read_lock();
     620         453 :         owner = rwsem_owner_flags(sem, &flags);
     621             :         /*
     622             :          * Don't check the read-owner as the entry may be stale.
     623             :          */
     624         453 :         if ((flags & RWSEM_NONSPINNABLE) ||
     625         313 :             (owner && !(flags & RWSEM_READER_OWNED) && !owner_on_cpu(owner)))
     626             :                 ret = false;
     627         453 :         rcu_read_unlock();
     628         453 :         preempt_enable();
     629             : 
     630         453 :         lockevent_cond_inc(rwsem_opt_fail, !ret);
     631         453 :         return ret;
     632             : }
     633             : 
     634             : /*
     635             :  * The rwsem_spin_on_owner() function returns the folowing 4 values
     636             :  * depending on the lock owner state.
     637             :  *   OWNER_NULL  : owner is currently NULL
     638             :  *   OWNER_WRITER: when owner changes and is a writer
     639             :  *   OWNER_READER: when owner changes and the new owner may be a reader.
     640             :  *   OWNER_NONSPINNABLE:
     641             :  *                 when optimistic spinning has to stop because either the
     642             :  *                 owner stops running, is unknown, or its timeslice has
     643             :  *                 been used up.
     644             :  */
     645             : enum owner_state {
     646             :         OWNER_NULL              = 1 << 0,
     647             :         OWNER_WRITER            = 1 << 1,
     648             :         OWNER_READER            = 1 << 2,
     649             :         OWNER_NONSPINNABLE      = 1 << 3,
     650             : };
     651             : #define OWNER_SPINNABLE         (OWNER_NULL | OWNER_WRITER | OWNER_READER)
     652             : 
     653             : static inline enum owner_state
     654        3168 : rwsem_owner_state(struct task_struct *owner, unsigned long flags)
     655             : {
     656        3168 :         if (flags & RWSEM_NONSPINNABLE)
     657             :                 return OWNER_NONSPINNABLE;
     658             : 
     659        3168 :         if (flags & RWSEM_READER_OWNED)
     660             :                 return OWNER_READER;
     661             : 
     662         637 :         return owner ? OWNER_WRITER : OWNER_NULL;
     663             : }
     664             : 
     665             : static noinline enum owner_state
     666        2909 : rwsem_spin_on_owner(struct rw_semaphore *sem)
     667             : {
     668        2909 :         struct task_struct *new, *owner;
     669        2909 :         unsigned long flags, new_flags;
     670        2909 :         enum owner_state state;
     671             : 
     672        2909 :         owner = rwsem_owner_flags(sem, &flags);
     673        2909 :         state = rwsem_owner_state(owner, flags);
     674         271 :         if (state != OWNER_WRITER)
     675        2638 :                 return state;
     676             : 
     677         271 :         rcu_read_lock();
     678      222062 :         for (;;) {
     679             :                 /*
     680             :                  * When a waiting writer set the handoff flag, it may spin
     681             :                  * on the owner as well. Once that writer acquires the lock,
     682             :                  * we can spin on it. So we don't need to quit even when the
     683             :                  * handoff bit is set.
     684             :                  */
     685      222062 :                 new = rwsem_owner_flags(sem, &new_flags);
     686      222062 :                 if ((new != owner) || (new_flags != flags)) {
     687         259 :                         state = rwsem_owner_state(new, new_flags);
     688             :                         break;
     689             :                 }
     690             : 
     691             :                 /*
     692             :                  * Ensure we emit the owner->on_cpu, dereference _after_
     693             :                  * checking sem->owner still matches owner, if that fails,
     694             :                  * owner might point to free()d memory, if it still matches,
     695             :                  * the rcu_read_lock() ensures the memory stays valid.
     696             :                  */
     697      221803 :                 barrier();
     698             : 
     699      221803 :                 if (need_resched() || !owner_on_cpu(owner)) {
     700             :                         state = OWNER_NONSPINNABLE;
     701             :                         break;
     702             :                 }
     703             : 
     704      221791 :                 cpu_relax();
     705             :         }
     706         271 :         rcu_read_unlock();
     707             : 
     708         271 :         return state;
     709             : }
     710             : 
     711             : /*
     712             :  * Calculate reader-owned rwsem spinning threshold for writer
     713             :  *
     714             :  * The more readers own the rwsem, the longer it will take for them to
     715             :  * wind down and free the rwsem. So the empirical formula used to
     716             :  * determine the actual spinning time limit here is:
     717             :  *
     718             :  *   Spinning threshold = (10 + nr_readers/2)us
     719             :  *
     720             :  * The limit is capped to a maximum of 25us (30 readers). This is just
     721             :  * a heuristic and is subjected to change in the future.
     722             :  */
     723          32 : static inline u64 rwsem_rspin_threshold(struct rw_semaphore *sem)
     724             : {
     725          32 :         long count = atomic_long_read(&sem->count);
     726          32 :         int readers = count >> RWSEM_READER_SHIFT;
     727          32 :         u64 delta;
     728             : 
     729          32 :         if (readers > 30)
     730             :                 readers = 30;
     731          32 :         delta = (20 + readers) * NSEC_PER_USEC / 2;
     732             : 
     733          32 :         return sched_clock() + delta;
     734             : }
     735             : 
     736         424 : static bool rwsem_optimistic_spin(struct rw_semaphore *sem)
     737             : {
     738         424 :         bool taken = false;
     739         424 :         int prev_owner_state = OWNER_NULL;
     740         424 :         int loop = 0;
     741         424 :         u64 rspin_threshold = 0;
     742             : 
     743         424 :         preempt_disable();
     744             : 
     745             :         /* sem->wait_lock should not be held when doing optimistic spinning */
     746         424 :         if (!osq_lock(&sem->osq))
     747           0 :                 goto done;
     748             : 
     749             :         /*
     750             :          * Optimistically spin on the owner field and attempt to acquire the
     751             :          * lock whenever the owner changes. Spinning will be stopped when:
     752             :          *  1) the owning writer isn't running; or
     753             :          *  2) readers own the lock and spinning time has exceeded limit.
     754             :          */
     755        2905 :         for (;;) {
     756        2905 :                 enum owner_state owner_state;
     757             : 
     758        2905 :                 owner_state = rwsem_spin_on_owner(sem);
     759        2905 :                 if (!(owner_state & OWNER_SPINNABLE))
     760             :                         break;
     761             : 
     762             :                 /*
     763             :                  * Try to acquire the lock
     764             :                  */
     765        2894 :                 taken = rwsem_try_write_lock_unqueued(sem);
     766             : 
     767        2894 :                 if (taken)
     768             :                         break;
     769             : 
     770             :                 /*
     771             :                  * Time-based reader-owned rwsem optimistic spinning
     772             :                  */
     773        2513 :                 if (owner_state == OWNER_READER) {
     774             :                         /*
     775             :                          * Re-initialize rspin_threshold every time when
     776             :                          * the owner state changes from non-reader to reader.
     777             :                          * This allows a writer to steal the lock in between
     778             :                          * 2 reader phases and have the threshold reset at
     779             :                          * the beginning of the 2nd reader phase.
     780             :                          */
     781        2512 :                         if (prev_owner_state != OWNER_READER) {
     782          32 :                                 if (rwsem_test_oflags(sem, RWSEM_NONSPINNABLE))
     783             :                                         break;
     784          32 :                                 rspin_threshold = rwsem_rspin_threshold(sem);
     785          32 :                                 loop = 0;
     786             :                         }
     787             : 
     788             :                         /*
     789             :                          * Check time threshold once every 16 iterations to
     790             :                          * avoid calling sched_clock() too frequently so
     791             :                          * as to reduce the average latency between the times
     792             :                          * when the lock becomes free and when the spinner
     793             :                          * is ready to do a trylock.
     794             :                          */
     795        2480 :                         else if (!(++loop & 0xf) && (sched_clock() > rspin_threshold)) {
     796          32 :                                 rwsem_set_nonspinnable(sem);
     797          32 :                                 lockevent_inc(rwsem_opt_nospin);
     798          32 :                                 break;
     799             :                         }
     800             :                 }
     801             : 
     802             :                 /*
     803             :                  * An RT task cannot do optimistic spinning if it cannot
     804             :                  * be sure the lock holder is running or live-lock may
     805             :                  * happen if the current task and the lock holder happen
     806             :                  * to run in the same CPU. However, aborting optimistic
     807             :                  * spinning while a NULL owner is detected may miss some
     808             :                  * opportunity where spinning can continue without causing
     809             :                  * problem.
     810             :                  *
     811             :                  * There are 2 possible cases where an RT task may be able
     812             :                  * to continue spinning.
     813             :                  *
     814             :                  * 1) The lock owner is in the process of releasing the
     815             :                  *    lock, sem->owner is cleared but the lock has not
     816             :                  *    been released yet.
     817             :                  * 2) The lock was free and owner cleared, but another
     818             :                  *    task just comes in and acquire the lock before
     819             :                  *    we try to get it. The new owner may be a spinnable
     820             :                  *    writer.
     821             :                  *
     822             :                  * To take advantage of two scenarios listed agove, the RT
     823             :                  * task is made to retry one more time to see if it can
     824             :                  * acquire the lock or continue spinning on the new owning
     825             :                  * writer. Of course, if the time lag is long enough or the
     826             :                  * new owner is not a writer or spinnable, the RT task will
     827             :                  * quit spinning.
     828             :                  *
     829             :                  * If the owner is a writer, the need_resched() check is
     830             :                  * done inside rwsem_spin_on_owner(). If the owner is not
     831             :                  * a writer, need_resched() check needs to be done here.
     832             :                  */
     833        2481 :                 if (owner_state != OWNER_WRITER) {
     834        2481 :                         if (need_resched())
     835             :                                 break;
     836        2481 :                         if (rt_task(current) &&
     837             :                            (prev_owner_state != OWNER_WRITER))
     838             :                                 break;
     839             :                 }
     840        2481 :                 prev_owner_state = owner_state;
     841             : 
     842             :                 /*
     843             :                  * The cpu_relax() call is a compiler barrier which forces
     844             :                  * everything in this loop to be re-loaded. We don't need
     845             :                  * memory barriers as we'll eventually observe the right
     846             :                  * values at the cost of a few extra spins.
     847             :                  */
     848        2481 :                 cpu_relax();
     849             :         }
     850         424 :         osq_unlock(&sem->osq);
     851         424 : done:
     852         424 :         preempt_enable();
     853         424 :         lockevent_cond_inc(rwsem_opt_fail, !taken);
     854         424 :         return taken;
     855             : }
     856             : 
     857             : /*
     858             :  * Clear the owner's RWSEM_NONSPINNABLE bit if it is set. This should
     859             :  * only be called when the reader count reaches 0.
     860             :  */
     861          71 : static inline void clear_nonspinnable(struct rw_semaphore *sem)
     862             : {
     863          71 :         if (rwsem_test_oflags(sem, RWSEM_NONSPINNABLE))
     864          32 :                 atomic_long_andnot(RWSEM_NONSPINNABLE, &sem->owner);
     865          71 : }
     866             : 
     867             : #else
     868             : static inline bool rwsem_can_spin_on_owner(struct rw_semaphore *sem)
     869             : {
     870             :         return false;
     871             : }
     872             : 
     873             : static inline bool rwsem_optimistic_spin(struct rw_semaphore *sem)
     874             : {
     875             :         return false;
     876             : }
     877             : 
     878             : static inline void clear_nonspinnable(struct rw_semaphore *sem) { }
     879             : 
     880             : static inline int
     881             : rwsem_spin_on_owner(struct rw_semaphore *sem)
     882             : {
     883             :         return 0;
     884             : }
     885             : #define OWNER_NULL      1
     886             : #endif
     887             : 
     888             : /*
     889             :  * Wait for the read lock to be granted
     890             :  */
     891             : static struct rw_semaphore __sched *
     892         110 : rwsem_down_read_slowpath(struct rw_semaphore *sem, long count, int state)
     893             : {
     894         110 :         long adjustment = -RWSEM_READER_BIAS;
     895         110 :         long rcnt = (count >> RWSEM_READER_SHIFT);
     896         110 :         struct rwsem_waiter waiter;
     897         110 :         DEFINE_WAKE_Q(wake_q);
     898         110 :         bool wake = false;
     899             : 
     900             :         /*
     901             :          * To prevent a constant stream of readers from starving a sleeping
     902             :          * waiter, don't attempt optimistic lock stealing if the lock is
     903             :          * currently owned by readers.
     904             :          */
     905         110 :         if ((atomic_long_read(&sem->owner) & RWSEM_READER_OWNED) &&
     906          36 :             (rcnt > 1) && !(count & RWSEM_WRITER_LOCKED))
     907          36 :                 goto queue;
     908             : 
     909             :         /*
     910             :          * Reader optimistic lock stealing.
     911             :          */
     912          74 :         if (!(count & (RWSEM_WRITER_LOCKED | RWSEM_FLAG_HANDOFF))) {
     913          37 :                 rwsem_set_reader_owned(sem);
     914          37 :                 lockevent_inc(rwsem_rlock_steal);
     915             : 
     916             :                 /*
     917             :                  * Wake up other readers in the wait queue if it is
     918             :                  * the first reader.
     919             :                  */
     920          37 :                 if ((rcnt == 1) && (count & RWSEM_FLAG_WAITERS)) {
     921          37 :                         raw_spin_lock_irq(&sem->wait_lock);
     922          37 :                         if (!list_empty(&sem->wait_list))
     923          37 :                                 rwsem_mark_wake(sem, RWSEM_WAKE_READ_OWNED,
     924             :                                                 &wake_q);
     925          37 :                         raw_spin_unlock_irq(&sem->wait_lock);
     926          37 :                         wake_up_q(&wake_q);
     927             :                 }
     928          37 :                 return sem;
     929             :         }
     930             : 
     931          37 : queue:
     932          73 :         waiter.task = current;
     933          73 :         waiter.type = RWSEM_WAITING_FOR_READ;
     934          73 :         waiter.timeout = jiffies + RWSEM_WAIT_TIMEOUT;
     935             : 
     936          73 :         raw_spin_lock_irq(&sem->wait_lock);
     937          73 :         if (list_empty(&sem->wait_list)) {
     938             :                 /*
     939             :                  * In case the wait queue is empty and the lock isn't owned
     940             :                  * by a writer or has the handoff bit set, this reader can
     941             :                  * exit the slowpath and return immediately as its
     942             :                  * RWSEM_READER_BIAS has already been set in the count.
     943             :                  */
     944          24 :                 if (!(atomic_long_read(&sem->count) &
     945             :                      (RWSEM_WRITER_MASK | RWSEM_FLAG_HANDOFF))) {
     946             :                         /* Provide lock ACQUIRE */
     947           0 :                         smp_acquire__after_ctrl_dep();
     948           0 :                         raw_spin_unlock_irq(&sem->wait_lock);
     949           0 :                         rwsem_set_reader_owned(sem);
     950           0 :                         lockevent_inc(rwsem_rlock_fast);
     951           0 :                         return sem;
     952             :                 }
     953             :                 adjustment += RWSEM_FLAG_WAITERS;
     954             :         }
     955          73 :         list_add_tail(&waiter.list, &sem->wait_list);
     956             : 
     957             :         /* we're now waiting on the lock, but no longer actively locking */
     958          73 :         count = atomic_long_add_return(adjustment, &sem->count);
     959             : 
     960             :         /*
     961             :          * If there are no active locks, wake the front queued process(es).
     962             :          *
     963             :          * If there are no writers and we are first in the queue,
     964             :          * wake our own waiter to join the existing active readers !
     965             :          */
     966          73 :         if (!(count & RWSEM_LOCK_MASK)) {
     967           0 :                 clear_nonspinnable(sem);
     968           0 :                 wake = true;
     969             :         }
     970          73 :         if (wake || (!(count & RWSEM_WRITER_MASK) &&
     971          36 :                     (adjustment & RWSEM_FLAG_WAITERS)))
     972           0 :                 rwsem_mark_wake(sem, RWSEM_WAKE_ANY, &wake_q);
     973             : 
     974          73 :         raw_spin_unlock_irq(&sem->wait_lock);
     975          73 :         wake_up_q(&wake_q);
     976             : 
     977             :         /* wait to be given the lock */
     978         219 :         for (;;) {
     979         146 :                 set_current_state(state);
     980         146 :                 if (!smp_load_acquire(&waiter.task)) {
     981             :                         /* Matches rwsem_mark_wake()'s smp_store_release(). */
     982             :                         break;
     983             :                 }
     984          73 :                 if (signal_pending_state(state, current)) {
     985           0 :                         raw_spin_lock_irq(&sem->wait_lock);
     986           0 :                         if (waiter.task)
     987           0 :                                 goto out_nolock;
     988           0 :                         raw_spin_unlock_irq(&sem->wait_lock);
     989             :                         /* Ordered by sem->wait_lock against rwsem_mark_wake(). */
     990           0 :                         break;
     991             :                 }
     992          73 :                 schedule();
     993          73 :                 lockevent_inc(rwsem_sleep_reader);
     994             :         }
     995             : 
     996          73 :         __set_current_state(TASK_RUNNING);
     997          73 :         lockevent_inc(rwsem_rlock);
     998          73 :         return sem;
     999             : 
    1000           0 : out_nolock:
    1001           0 :         list_del(&waiter.list);
    1002           0 :         if (list_empty(&sem->wait_list)) {
    1003           0 :                 atomic_long_andnot(RWSEM_FLAG_WAITERS|RWSEM_FLAG_HANDOFF,
    1004             :                                    &sem->count);
    1005             :         }
    1006           0 :         raw_spin_unlock_irq(&sem->wait_lock);
    1007           0 :         __set_current_state(TASK_RUNNING);
    1008           0 :         lockevent_inc(rwsem_rlock_fail);
    1009           0 :         return ERR_PTR(-EINTR);
    1010             : }
    1011             : 
    1012             : /*
    1013             :  * Wait until we successfully acquire the write lock
    1014             :  */
    1015             : static struct rw_semaphore *
    1016         470 : rwsem_down_write_slowpath(struct rw_semaphore *sem, int state)
    1017             : {
    1018         470 :         long count;
    1019         470 :         enum writer_wait_state wstate;
    1020         470 :         struct rwsem_waiter waiter;
    1021         470 :         struct rw_semaphore *ret = sem;
    1022         470 :         DEFINE_WAKE_Q(wake_q);
    1023             : 
    1024             :         /* do optimistic spinning and steal lock if possible */
    1025         470 :         if (rwsem_can_spin_on_owner(sem) && rwsem_optimistic_spin(sem)) {
    1026             :                 /* rwsem_optimistic_spin() implies ACQUIRE on success */
    1027             :                 return sem;
    1028             :         }
    1029             : 
    1030             :         /*
    1031             :          * Optimistic spinning failed, proceed to the slowpath
    1032             :          * and block until we can acquire the sem.
    1033             :          */
    1034          89 :         waiter.task = current;
    1035          89 :         waiter.type = RWSEM_WAITING_FOR_WRITE;
    1036          89 :         waiter.timeout = jiffies + RWSEM_WAIT_TIMEOUT;
    1037             : 
    1038          89 :         raw_spin_lock_irq(&sem->wait_lock);
    1039             : 
    1040             :         /* account for this before adding a new element to the list */
    1041          89 :         wstate = list_empty(&sem->wait_list) ? WRITER_FIRST : WRITER_NOT_FIRST;
    1042             : 
    1043          89 :         list_add_tail(&waiter.list, &sem->wait_list);
    1044             : 
    1045             :         /* we're now waiting on the lock */
    1046          89 :         if (wstate == WRITER_NOT_FIRST) {
    1047          29 :                 count = atomic_long_read(&sem->count);
    1048             : 
    1049             :                 /*
    1050             :                  * If there were already threads queued before us and:
    1051             :                  *  1) there are no active locks, wake the front
    1052             :                  *     queued process(es) as the handoff bit might be set.
    1053             :                  *  2) there are no active writers and some readers, the lock
    1054             :                  *     must be read owned; so we try to wake any read lock
    1055             :                  *     waiters that were queued ahead of us.
    1056             :                  */
    1057          29 :                 if (count & RWSEM_WRITER_MASK)
    1058          11 :                         goto wait;
    1059             : 
    1060          18 :                 rwsem_mark_wake(sem, (count & RWSEM_READER_MASK)
    1061             :                                         ? RWSEM_WAKE_READERS
    1062             :                                         : RWSEM_WAKE_ANY, &wake_q);
    1063             : 
    1064          18 :                 if (!wake_q_empty(&wake_q)) {
    1065             :                         /*
    1066             :                          * We want to minimize wait_lock hold time especially
    1067             :                          * when a large number of readers are to be woken up.
    1068             :                          */
    1069           5 :                         raw_spin_unlock_irq(&sem->wait_lock);
    1070           5 :                         wake_up_q(&wake_q);
    1071           5 :                         wake_q_init(&wake_q);       /* Used again, reinit */
    1072           5 :                         raw_spin_lock_irq(&sem->wait_lock);
    1073             :                 }
    1074             :         } else {
    1075          60 :                 atomic_long_or(RWSEM_FLAG_WAITERS, &sem->count);
    1076             :         }
    1077             : 
    1078          89 : wait:
    1079             :         /* wait until we successfully acquire the lock */
    1080          89 :         set_current_state(state);
    1081         251 :         for (;;) {
    1082         170 :                 if (rwsem_try_write_lock(sem, wstate)) {
    1083             :                         /* rwsem_try_write_lock() implies ACQUIRE on success */
    1084             :                         break;
    1085             :                 }
    1086             : 
    1087          81 :                 raw_spin_unlock_irq(&sem->wait_lock);
    1088             : 
    1089             :                 /*
    1090             :                  * After setting the handoff bit and failing to acquire
    1091             :                  * the lock, attempt to spin on owner to accelerate lock
    1092             :                  * transfer. If the previous owner is a on-cpu writer and it
    1093             :                  * has just released the lock, OWNER_NULL will be returned.
    1094             :                  * In this case, we attempt to acquire the lock again
    1095             :                  * without sleeping.
    1096             :                  */
    1097          85 :                 if (wstate == WRITER_HANDOFF &&
    1098           4 :                     rwsem_spin_on_owner(sem) == OWNER_NULL)
    1099           1 :                         goto trylock_again;
    1100             : 
    1101             :                 /* Block until there are no active lockers. */
    1102          82 :                 for (;;) {
    1103          82 :                         if (signal_pending_state(state, current))
    1104           0 :                                 goto out_nolock;
    1105             : 
    1106          82 :                         schedule();
    1107          82 :                         lockevent_inc(rwsem_sleep_writer);
    1108          82 :                         set_current_state(state);
    1109             :                         /*
    1110             :                          * If HANDOFF bit is set, unconditionally do
    1111             :                          * a trylock.
    1112             :                          */
    1113          82 :                         if (wstate == WRITER_HANDOFF)
    1114             :                                 break;
    1115             : 
    1116          79 :                         if ((wstate == WRITER_NOT_FIRST) &&
    1117          24 :                             (rwsem_first_waiter(sem) == &waiter))
    1118          24 :                                 wstate = WRITER_FIRST;
    1119             : 
    1120          79 :                         count = atomic_long_read(&sem->count);
    1121          79 :                         if (!(count & RWSEM_LOCK_MASK))
    1122             :                                 break;
    1123             : 
    1124             :                         /*
    1125             :                          * The setting of the handoff bit is deferred
    1126             :                          * until rwsem_try_write_lock() is called.
    1127             :                          */
    1128           6 :                         if ((wstate == WRITER_FIRST) && (rt_task(current) ||
    1129           6 :                             time_after(jiffies, waiter.timeout))) {
    1130             :                                 wstate = WRITER_HANDOFF;
    1131             :                                 lockevent_inc(rwsem_wlock_handoff);
    1132             :                                 break;
    1133             :                         }
    1134             :                 }
    1135          81 : trylock_again:
    1136          81 :                 raw_spin_lock_irq(&sem->wait_lock);
    1137             :         }
    1138          89 :         __set_current_state(TASK_RUNNING);
    1139          89 :         list_del(&waiter.list);
    1140          89 :         raw_spin_unlock_irq(&sem->wait_lock);
    1141          89 :         lockevent_inc(rwsem_wlock);
    1142             : 
    1143          89 :         return ret;
    1144             : 
    1145           0 : out_nolock:
    1146           0 :         __set_current_state(TASK_RUNNING);
    1147           0 :         raw_spin_lock_irq(&sem->wait_lock);
    1148           0 :         list_del(&waiter.list);
    1149             : 
    1150           0 :         if (unlikely(wstate == WRITER_HANDOFF))
    1151           0 :                 atomic_long_add(-RWSEM_FLAG_HANDOFF,  &sem->count);
    1152             : 
    1153           0 :         if (list_empty(&sem->wait_list))
    1154           0 :                 atomic_long_andnot(RWSEM_FLAG_WAITERS, &sem->count);
    1155             :         else
    1156           0 :                 rwsem_mark_wake(sem, RWSEM_WAKE_ANY, &wake_q);
    1157           0 :         raw_spin_unlock_irq(&sem->wait_lock);
    1158           0 :         wake_up_q(&wake_q);
    1159           0 :         lockevent_inc(rwsem_wlock_fail);
    1160             : 
    1161           0 :         return ERR_PTR(-EINTR);
    1162             : }
    1163             : 
    1164             : /*
    1165             :  * handle waking up a waiter on the semaphore
    1166             :  * - up_read/up_write has decremented the active part of count if we come here
    1167             :  */
    1168         232 : static struct rw_semaphore *rwsem_wake(struct rw_semaphore *sem, long count)
    1169             : {
    1170         232 :         unsigned long flags;
    1171         232 :         DEFINE_WAKE_Q(wake_q);
    1172             : 
    1173         232 :         raw_spin_lock_irqsave(&sem->wait_lock, flags);
    1174             : 
    1175         232 :         if (!list_empty(&sem->wait_list))
    1176         232 :                 rwsem_mark_wake(sem, RWSEM_WAKE_ANY, &wake_q);
    1177             : 
    1178         232 :         raw_spin_unlock_irqrestore(&sem->wait_lock, flags);
    1179         232 :         wake_up_q(&wake_q);
    1180             : 
    1181         232 :         return sem;
    1182             : }
    1183             : 
    1184             : /*
    1185             :  * downgrade a write lock into a read lock
    1186             :  * - caller incremented waiting part of count and discovered it still negative
    1187             :  * - just wake up any readers at the front of the queue
    1188             :  */
    1189           0 : static struct rw_semaphore *rwsem_downgrade_wake(struct rw_semaphore *sem)
    1190             : {
    1191           0 :         unsigned long flags;
    1192           0 :         DEFINE_WAKE_Q(wake_q);
    1193             : 
    1194           0 :         raw_spin_lock_irqsave(&sem->wait_lock, flags);
    1195             : 
    1196           0 :         if (!list_empty(&sem->wait_list))
    1197           0 :                 rwsem_mark_wake(sem, RWSEM_WAKE_READ_OWNED, &wake_q);
    1198             : 
    1199           0 :         raw_spin_unlock_irqrestore(&sem->wait_lock, flags);
    1200           0 :         wake_up_q(&wake_q);
    1201             : 
    1202           0 :         return sem;
    1203             : }
    1204             : 
    1205             : /*
    1206             :  * lock for reading
    1207             :  */
    1208       35892 : static inline int __down_read_common(struct rw_semaphore *sem, int state)
    1209             : {
    1210       35892 :         long count;
    1211             : 
    1212       35892 :         if (!rwsem_read_trylock(sem, &count)) {
    1213         110 :                 if (IS_ERR(rwsem_down_read_slowpath(sem, count, state)))
    1214             :                         return -EINTR;
    1215         110 :                 DEBUG_RWSEMS_WARN_ON(!is_rwsem_reader_owned(sem), sem);
    1216             :         }
    1217             :         return 0;
    1218             : }
    1219             : 
    1220       33022 : static inline void __down_read(struct rw_semaphore *sem)
    1221             : {
    1222       33022 :         __down_read_common(sem, TASK_UNINTERRUPTIBLE);
    1223             : }
    1224             : 
    1225           0 : static inline int __down_read_interruptible(struct rw_semaphore *sem)
    1226             : {
    1227           0 :         return __down_read_common(sem, TASK_INTERRUPTIBLE);
    1228             : }
    1229             : 
    1230        2870 : static inline int __down_read_killable(struct rw_semaphore *sem)
    1231             : {
    1232        2870 :         return __down_read_common(sem, TASK_KILLABLE);
    1233             : }
    1234             : 
    1235      144506 : static inline int __down_read_trylock(struct rw_semaphore *sem)
    1236             : {
    1237      144506 :         long tmp;
    1238             : 
    1239      144506 :         DEBUG_RWSEMS_WARN_ON(sem->magic != sem, sem);
    1240             : 
    1241             :         /*
    1242             :          * Optimize for the case when the rwsem is not locked at all.
    1243             :          */
    1244      144506 :         tmp = RWSEM_UNLOCKED_VALUE;
    1245      144510 :         do {
    1246      144521 :                 if (atomic_long_try_cmpxchg_acquire(&sem->count, &tmp,
    1247      144510 :                                         tmp + RWSEM_READER_BIAS)) {
    1248      144419 :                         rwsem_set_reader_owned(sem);
    1249      144419 :                         return 1;
    1250             :                 }
    1251         102 :         } while (!(tmp & RWSEM_READ_FAILED_MASK));
    1252             :         return 0;
    1253             : }
    1254             : 
    1255             : /*
    1256             :  * lock for writing
    1257             :  */
    1258      328803 : static inline int __down_write_common(struct rw_semaphore *sem, int state)
    1259             : {
    1260      328803 :         if (unlikely(!rwsem_write_trylock(sem))) {
    1261         470 :                 if (IS_ERR(rwsem_down_write_slowpath(sem, state)))
    1262           0 :                         return -EINTR;
    1263             :         }
    1264             : 
    1265             :         return 0;
    1266             : }
    1267             : 
    1268      287725 : static inline void __down_write(struct rw_semaphore *sem)
    1269             : {
    1270      287725 :         __down_write_common(sem, TASK_UNINTERRUPTIBLE);
    1271             : }
    1272             : 
    1273       41096 : static inline int __down_write_killable(struct rw_semaphore *sem)
    1274             : {
    1275       41096 :         return __down_write_common(sem, TASK_KILLABLE);
    1276             : }
    1277             : 
    1278           0 : static inline int __down_write_trylock(struct rw_semaphore *sem)
    1279             : {
    1280           0 :         DEBUG_RWSEMS_WARN_ON(sem->magic != sem, sem);
    1281           0 :         return rwsem_write_trylock(sem);
    1282             : }
    1283             : 
    1284             : /*
    1285             :  * unlock after reading
    1286             :  */
    1287      181426 : static inline void __up_read(struct rw_semaphore *sem)
    1288             : {
    1289      181426 :         long tmp;
    1290             : 
    1291      181426 :         DEBUG_RWSEMS_WARN_ON(sem->magic != sem, sem);
    1292      181426 :         DEBUG_RWSEMS_WARN_ON(!is_rwsem_reader_owned(sem), sem);
    1293             : 
    1294      181428 :         rwsem_clear_reader_owned(sem);
    1295      181456 :         tmp = atomic_long_add_return_release(-RWSEM_READER_BIAS, &sem->count);
    1296      181461 :         DEBUG_RWSEMS_WARN_ON(tmp < 0, sem);
    1297      181461 :         if (unlikely((tmp & (RWSEM_LOCK_MASK|RWSEM_FLAG_WAITERS)) ==
    1298             :                       RWSEM_FLAG_WAITERS)) {
    1299          71 :                 clear_nonspinnable(sem);
    1300          71 :                 rwsem_wake(sem, tmp);
    1301             :         }
    1302      181461 : }
    1303             : 
    1304             : /*
    1305             :  * unlock after writing
    1306             :  */
    1307      327639 : static inline void __up_write(struct rw_semaphore *sem)
    1308             : {
    1309      327639 :         long tmp;
    1310             : 
    1311      327639 :         DEBUG_RWSEMS_WARN_ON(sem->magic != sem, sem);
    1312             :         /*
    1313             :          * sem->owner may differ from current if the ownership is transferred
    1314             :          * to an anonymous writer by setting the RWSEM_NONSPINNABLE bits.
    1315             :          */
    1316      327639 :         DEBUG_RWSEMS_WARN_ON((rwsem_owner(sem) != current) &&
    1317             :                             !rwsem_test_oflags(sem, RWSEM_NONSPINNABLE), sem);
    1318             : 
    1319      327639 :         rwsem_clear_owner(sem);
    1320      327660 :         tmp = atomic_long_fetch_add_release(-RWSEM_WRITER_LOCKED, &sem->count);
    1321      327725 :         if (unlikely(tmp & RWSEM_FLAG_WAITERS))
    1322         161 :                 rwsem_wake(sem, tmp);
    1323      327725 : }
    1324             : 
    1325             : /*
    1326             :  * downgrade write lock to read lock
    1327             :  */
    1328        1149 : static inline void __downgrade_write(struct rw_semaphore *sem)
    1329             : {
    1330        1149 :         long tmp;
    1331             : 
    1332             :         /*
    1333             :          * When downgrading from exclusive to shared ownership,
    1334             :          * anything inside the write-locked region cannot leak
    1335             :          * into the read side. In contrast, anything in the
    1336             :          * read-locked region is ok to be re-ordered into the
    1337             :          * write side. As such, rely on RELEASE semantics.
    1338             :          */
    1339        1149 :         DEBUG_RWSEMS_WARN_ON(rwsem_owner(sem) != current, sem);
    1340        1149 :         tmp = atomic_long_fetch_add_release(
    1341             :                 -RWSEM_WRITER_LOCKED+RWSEM_READER_BIAS, &sem->count);
    1342        1149 :         rwsem_set_reader_owned(sem);
    1343        1149 :         if (tmp & RWSEM_FLAG_WAITERS)
    1344           0 :                 rwsem_downgrade_wake(sem);
    1345        1149 : }
    1346             : 
    1347             : /*
    1348             :  * lock for reading
    1349             :  */
    1350       33022 : void __sched down_read(struct rw_semaphore *sem)
    1351             : {
    1352       33022 :         might_sleep();
    1353       33023 :         rwsem_acquire_read(&sem->dep_map, 0, 0, _RET_IP_);
    1354             : 
    1355       33022 :         LOCK_CONTENDED(sem, __down_read_trylock, __down_read);
    1356       33024 : }
    1357             : EXPORT_SYMBOL(down_read);
    1358             : 
    1359           0 : int __sched down_read_interruptible(struct rw_semaphore *sem)
    1360             : {
    1361           0 :         might_sleep();
    1362           0 :         rwsem_acquire_read(&sem->dep_map, 0, 0, _RET_IP_);
    1363             : 
    1364           0 :         if (LOCK_CONTENDED_RETURN(sem, __down_read_trylock, __down_read_interruptible)) {
    1365           0 :                 rwsem_release(&sem->dep_map, _RET_IP_);
    1366           0 :                 return -EINTR;
    1367             :         }
    1368             : 
    1369             :         return 0;
    1370             : }
    1371             : EXPORT_SYMBOL(down_read_interruptible);
    1372             : 
    1373        2870 : int __sched down_read_killable(struct rw_semaphore *sem)
    1374             : {
    1375        2870 :         might_sleep();
    1376        2870 :         rwsem_acquire_read(&sem->dep_map, 0, 0, _RET_IP_);
    1377             : 
    1378        2870 :         if (LOCK_CONTENDED_RETURN(sem, __down_read_trylock, __down_read_killable)) {
    1379           0 :                 rwsem_release(&sem->dep_map, _RET_IP_);
    1380           0 :                 return -EINTR;
    1381             :         }
    1382             : 
    1383             :         return 0;
    1384             : }
    1385             : EXPORT_SYMBOL(down_read_killable);
    1386             : 
    1387             : /*
    1388             :  * trylock for reading -- returns 1 if successful, 0 if contention
    1389             :  */
    1390      144506 : int down_read_trylock(struct rw_semaphore *sem)
    1391             : {
    1392      144506 :         int ret = __down_read_trylock(sem);
    1393             : 
    1394      144510 :         if (ret == 1)
    1395      144412 :                 rwsem_acquire_read(&sem->dep_map, 0, 1, _RET_IP_);
    1396      144500 :         return ret;
    1397             : }
    1398             : EXPORT_SYMBOL(down_read_trylock);
    1399             : 
    1400             : /*
    1401             :  * lock for writing
    1402             :  */
    1403      279557 : void __sched down_write(struct rw_semaphore *sem)
    1404             : {
    1405      279557 :         might_sleep();
    1406      279575 :         rwsem_acquire(&sem->dep_map, 0, 0, _RET_IP_);
    1407      279544 :         LOCK_CONTENDED(sem, __down_write_trylock, __down_write);
    1408      279560 : }
    1409             : EXPORT_SYMBOL(down_write);
    1410             : 
    1411             : /*
    1412             :  * lock for writing
    1413             :  */
    1414       41098 : int __sched down_write_killable(struct rw_semaphore *sem)
    1415             : {
    1416       41098 :         might_sleep();
    1417       41098 :         rwsem_acquire(&sem->dep_map, 0, 0, _RET_IP_);
    1418             : 
    1419       41096 :         if (LOCK_CONTENDED_RETURN(sem, __down_write_trylock,
    1420             :                                   __down_write_killable)) {
    1421           0 :                 rwsem_release(&sem->dep_map, _RET_IP_);
    1422           0 :                 return -EINTR;
    1423             :         }
    1424             : 
    1425             :         return 0;
    1426             : }
    1427             : EXPORT_SYMBOL(down_write_killable);
    1428             : 
    1429             : /*
    1430             :  * trylock for writing -- returns 1 if successful, 0 if contention
    1431             :  */
    1432           0 : int down_write_trylock(struct rw_semaphore *sem)
    1433             : {
    1434           0 :         int ret = __down_write_trylock(sem);
    1435             : 
    1436           0 :         if (ret == 1)
    1437           0 :                 rwsem_acquire(&sem->dep_map, 0, 1, _RET_IP_);
    1438             : 
    1439           0 :         return ret;
    1440             : }
    1441             : EXPORT_SYMBOL(down_write_trylock);
    1442             : 
    1443             : /*
    1444             :  * release a read lock
    1445             :  */
    1446      181444 : void up_read(struct rw_semaphore *sem)
    1447             : {
    1448      181444 :         rwsem_release(&sem->dep_map, _RET_IP_);
    1449      181433 :         __up_read(sem);
    1450      181460 : }
    1451             : EXPORT_SYMBOL(up_read);
    1452             : 
    1453             : /*
    1454             :  * release a write lock
    1455             :  */
    1456      327673 : void up_write(struct rw_semaphore *sem)
    1457             : {
    1458      327673 :         rwsem_release(&sem->dep_map, _RET_IP_);
    1459      327641 :         __up_write(sem);
    1460      327721 : }
    1461             : EXPORT_SYMBOL(up_write);
    1462             : 
    1463             : /*
    1464             :  * downgrade write lock to read lock
    1465             :  */
    1466        1149 : void downgrade_write(struct rw_semaphore *sem)
    1467             : {
    1468        1149 :         lock_downgrade(&sem->dep_map, _RET_IP_);
    1469        1149 :         __downgrade_write(sem);
    1470        1149 : }
    1471             : EXPORT_SYMBOL(downgrade_write);
    1472             : 
    1473             : #ifdef CONFIG_DEBUG_LOCK_ALLOC
    1474             : 
    1475           0 : void down_read_nested(struct rw_semaphore *sem, int subclass)
    1476             : {
    1477           0 :         might_sleep();
    1478           0 :         rwsem_acquire_read(&sem->dep_map, subclass, 0, _RET_IP_);
    1479           0 :         LOCK_CONTENDED(sem, __down_read_trylock, __down_read);
    1480           0 : }
    1481             : EXPORT_SYMBOL(down_read_nested);
    1482             : 
    1483           0 : int down_read_killable_nested(struct rw_semaphore *sem, int subclass)
    1484             : {
    1485           0 :         might_sleep();
    1486           0 :         rwsem_acquire_read(&sem->dep_map, subclass, 0, _RET_IP_);
    1487             : 
    1488           0 :         if (LOCK_CONTENDED_RETURN(sem, __down_read_trylock, __down_read_killable)) {
    1489           0 :                 rwsem_release(&sem->dep_map, _RET_IP_);
    1490           0 :                 return -EINTR;
    1491             :         }
    1492             : 
    1493             :         return 0;
    1494             : }
    1495             : EXPORT_SYMBOL(down_read_killable_nested);
    1496             : 
    1497           0 : void _down_write_nest_lock(struct rw_semaphore *sem, struct lockdep_map *nest)
    1498             : {
    1499           0 :         might_sleep();
    1500           0 :         rwsem_acquire_nest(&sem->dep_map, 0, 0, nest, _RET_IP_);
    1501           0 :         LOCK_CONTENDED(sem, __down_write_trylock, __down_write);
    1502           0 : }
    1503             : EXPORT_SYMBOL(_down_write_nest_lock);
    1504             : 
    1505           0 : void down_read_non_owner(struct rw_semaphore *sem)
    1506             : {
    1507           0 :         might_sleep();
    1508           0 :         __down_read(sem);
    1509           0 :         __rwsem_set_reader_owned(sem, NULL);
    1510           0 : }
    1511             : EXPORT_SYMBOL(down_read_non_owner);
    1512             : 
    1513        8181 : void down_write_nested(struct rw_semaphore *sem, int subclass)
    1514             : {
    1515        8181 :         might_sleep();
    1516        8181 :         rwsem_acquire(&sem->dep_map, subclass, 0, _RET_IP_);
    1517        8181 :         LOCK_CONTENDED(sem, __down_write_trylock, __down_write);
    1518        8181 : }
    1519             : EXPORT_SYMBOL(down_write_nested);
    1520             : 
    1521           0 : int __sched down_write_killable_nested(struct rw_semaphore *sem, int subclass)
    1522             : {
    1523           0 :         might_sleep();
    1524           0 :         rwsem_acquire(&sem->dep_map, subclass, 0, _RET_IP_);
    1525             : 
    1526           0 :         if (LOCK_CONTENDED_RETURN(sem, __down_write_trylock,
    1527             :                                   __down_write_killable)) {
    1528           0 :                 rwsem_release(&sem->dep_map, _RET_IP_);
    1529           0 :                 return -EINTR;
    1530             :         }
    1531             : 
    1532             :         return 0;
    1533             : }
    1534             : EXPORT_SYMBOL(down_write_killable_nested);
    1535             : 
    1536           0 : void up_read_non_owner(struct rw_semaphore *sem)
    1537             : {
    1538           0 :         DEBUG_RWSEMS_WARN_ON(!is_rwsem_reader_owned(sem), sem);
    1539           0 :         __up_read(sem);
    1540           0 : }
    1541             : EXPORT_SYMBOL(up_read_non_owner);
    1542             : 
    1543             : #endif

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