Line data    Source code

       1             : /* SPDX-License-Identifier: GPL-2.0 */
       2             : /* rwsem.h: R/W semaphores, public interface
       3             :  *
       4             :  * Written by David Howells (dhowells@redhat.com).
       5             :  * Derived from asm-i386/semaphore.h
       6             :  */
       7             : 
       8             : #ifndef _LINUX_RWSEM_H
       9             : #define _LINUX_RWSEM_H
      10             : 
      11             : #include <linux/linkage.h>
      12             : 
      13             : #include <linux/types.h>
      14             : #include <linux/kernel.h>
      15             : #include <linux/list.h>
      16             : #include <linux/spinlock.h>
      17             : #include <linux/atomic.h>
      18             : #include <linux/err.h>
      19             : #ifdef CONFIG_RWSEM_SPIN_ON_OWNER
      20             : #include <linux/osq_lock.h>
      21             : #endif
      22             : 
      23             : /*
      24             :  * For an uncontended rwsem, count and owner are the only fields a task
      25             :  * needs to touch when acquiring the rwsem. So they are put next to each
      26             :  * other to increase the chance that they will share the same cacheline.
      27             :  *
      28             :  * In a contended rwsem, the owner is likely the most frequently accessed
      29             :  * field in the structure as the optimistic waiter that holds the osq lock
      30             :  * will spin on owner. For an embedded rwsem, other hot fields in the
      31             :  * containing structure should be moved further away from the rwsem to
      32             :  * reduce the chance that they will share the same cacheline causing
      33             :  * cacheline bouncing problem.
      34             :  */
      35             : struct rw_semaphore {
      36             :         atomic_long_t count;
      37             :         /*
      38             :          * Write owner or one of the read owners as well flags regarding
      39             :          * the current state of the rwsem. Can be used as a speculative
      40             :          * check to see if the write owner is running on the cpu.
      41             :          */
      42             :         atomic_long_t owner;
      43             : #ifdef CONFIG_RWSEM_SPIN_ON_OWNER
      44             :         struct optimistic_spin_queue osq; /* spinner MCS lock */
      45             : #endif
      46             :         raw_spinlock_t wait_lock;
      47             :         struct list_head wait_list;
      48             : #ifdef CONFIG_DEBUG_RWSEMS
      49             :         void *magic;
      50             : #endif
      51             : #ifdef CONFIG_DEBUG_LOCK_ALLOC
      52             :         struct lockdep_map      dep_map;
      53             : #endif
      54             : };
      55             : 
      56             : /* In all implementations count != 0 means locked */
      57       74378 : static inline int rwsem_is_locked(struct rw_semaphore *sem)
      58             : {
      59       74378 :         return atomic_long_read(&sem->count) != 0;
      60             : }
      61             : 
      62             : #define RWSEM_UNLOCKED_VALUE            0L
      63             : #define __RWSEM_COUNT_INIT(name)        .count = ATOMIC_LONG_INIT(RWSEM_UNLOCKED_VALUE)
      64             : 
      65             : /* Common initializer macros and functions */
      66             : 
      67             : #ifdef CONFIG_DEBUG_LOCK_ALLOC
      68             : # define __RWSEM_DEP_MAP_INIT(lockname)                 \
      69             :         .dep_map = {                                    \
      70             :                 .name = #lockname,                      \
      71             :                 .wait_type_inner = LD_WAIT_SLEEP,       \
      72             :         },
      73             : #else
      74             : # define __RWSEM_DEP_MAP_INIT(lockname)
      75             : #endif
      76             : 
      77             : #ifdef CONFIG_DEBUG_RWSEMS
      78             : # define __RWSEM_DEBUG_INIT(lockname) .magic = &lockname,
      79             : #else
      80             : # define __RWSEM_DEBUG_INIT(lockname)
      81             : #endif
      82             : 
      83             : #ifdef CONFIG_RWSEM_SPIN_ON_OWNER
      84             : #define __RWSEM_OPT_INIT(lockname) .osq = OSQ_LOCK_UNLOCKED,
      85             : #else
      86             : #define __RWSEM_OPT_INIT(lockname)
      87             : #endif
      88             : 
      89             : #define __RWSEM_INITIALIZER(name)                               \
      90             :         { __RWSEM_COUNT_INIT(name),                             \
      91             :           .owner = ATOMIC_LONG_INIT(0),                         \
      92             :           __RWSEM_OPT_INIT(name)                                \
      93             :           .wait_lock = __RAW_SPIN_LOCK_UNLOCKED(name.wait_lock),\
      94             :           .wait_list = LIST_HEAD_INIT((name).wait_list),        \
      95             :           __RWSEM_DEBUG_INIT(name)                              \
      96             :           __RWSEM_DEP_MAP_INIT(name) }
      97             : 
      98             : #define DECLARE_RWSEM(name) \
      99             :         struct rw_semaphore name = __RWSEM_INITIALIZER(name)
     100             : 
     101             : extern void __init_rwsem(struct rw_semaphore *sem, const char *name,
     102             :                          struct lock_class_key *key);
     103             : 
     104             : #define init_rwsem(sem)                                         \
     105             : do {                                                            \
     106             :         static struct lock_class_key __key;                     \
     107             :                                                                 \
     108             :         __init_rwsem((sem), #sem, &__key);                  \
     109             : } while (0)
     110             : 
     111             : /*
     112             :  * This is the same regardless of which rwsem implementation that is being used.
     113             :  * It is just a heuristic meant to be called by somebody alreadying holding the
     114             :  * rwsem to see if somebody from an incompatible type is wanting access to the
     115             :  * lock.
     116             :  */
     117           0 : static inline int rwsem_is_contended(struct rw_semaphore *sem)
     118             : {
     119           0 :         return !list_empty(&sem->wait_list);
     120             : }
     121             : 
     122             : /*
     123             :  * lock for reading
     124             :  */
     125             : extern void down_read(struct rw_semaphore *sem);
     126             : extern int __must_check down_read_interruptible(struct rw_semaphore *sem);
     127             : extern int __must_check down_read_killable(struct rw_semaphore *sem);
     128             : 
     129             : /*
     130             :  * trylock for reading -- returns 1 if successful, 0 if contention
     131             :  */
     132             : extern int down_read_trylock(struct rw_semaphore *sem);
     133             : 
     134             : /*
     135             :  * lock for writing
     136             :  */
     137             : extern void down_write(struct rw_semaphore *sem);
     138             : extern int __must_check down_write_killable(struct rw_semaphore *sem);
     139             : 
     140             : /*
     141             :  * trylock for writing -- returns 1 if successful, 0 if contention
     142             :  */
     143             : extern int down_write_trylock(struct rw_semaphore *sem);
     144             : 
     145             : /*
     146             :  * release a read lock
     147             :  */
     148             : extern void up_read(struct rw_semaphore *sem);
     149             : 
     150             : /*
     151             :  * release a write lock
     152             :  */
     153             : extern void up_write(struct rw_semaphore *sem);
     154             : 
     155             : /*
     156             :  * downgrade write lock to read lock
     157             :  */
     158             : extern void downgrade_write(struct rw_semaphore *sem);
     159             : 
     160             : #ifdef CONFIG_DEBUG_LOCK_ALLOC
     161             : /*
     162             :  * nested locking. NOTE: rwsems are not allowed to recurse
     163             :  * (which occurs if the same task tries to acquire the same
     164             :  * lock instance multiple times), but multiple locks of the
     165             :  * same lock class might be taken, if the order of the locks
     166             :  * is always the same. This ordering rule can be expressed
     167             :  * to lockdep via the _nested() APIs, but enumerating the
     168             :  * subclasses that are used. (If the nesting relationship is
     169             :  * static then another method for expressing nested locking is
     170             :  * the explicit definition of lock class keys and the use of
     171             :  * lockdep_set_class() at lock initialization time.
     172             :  * See Documentation/locking/lockdep-design.rst for more details.)
     173             :  */
     174             : extern void down_read_nested(struct rw_semaphore *sem, int subclass);
     175             : extern int __must_check down_read_killable_nested(struct rw_semaphore *sem, int subclass);
     176             : extern void down_write_nested(struct rw_semaphore *sem, int subclass);
     177             : extern int down_write_killable_nested(struct rw_semaphore *sem, int subclass);
     178             : extern void _down_write_nest_lock(struct rw_semaphore *sem, struct lockdep_map *nest_lock);
     179             : 
     180             : # define down_write_nest_lock(sem, nest_lock)                   \
     181             : do {                                                            \
     182             :         typecheck(struct lockdep_map *, &(nest_lock)->dep_map);  \
     183             :         _down_write_nest_lock(sem, &(nest_lock)->dep_map);       \
     184             : } while (0);
     185             : 
     186             : /*
     187             :  * Take/release a lock when not the owner will release it.
     188             :  *
     189             :  * [ This API should be avoided as much as possible - the
     190             :  *   proper abstraction for this case is completions. ]
     191             :  */
     192             : extern void down_read_non_owner(struct rw_semaphore *sem);
     193             : extern void up_read_non_owner(struct rw_semaphore *sem);
     194             : #else
     195             : # define down_read_nested(sem, subclass)                down_read(sem)
     196             : # define down_read_killable_nested(sem, subclass)       down_read_killable(sem)
     197             : # define down_write_nest_lock(sem, nest_lock)   down_write(sem)
     198             : # define down_write_nested(sem, subclass)       down_write(sem)
     199             : # define down_write_killable_nested(sem, subclass)      down_write_killable(sem)
     200             : # define down_read_non_owner(sem)               down_read(sem)
     201             : # define up_read_non_owner(sem)                 up_read(sem)
     202             : #endif
     203             : 
     204             : #endif /* _LINUX_RWSEM_H */