Line data    Source code

       1             : /* SPDX-License-Identifier: GPL-2.0 */
       2             : #ifndef _LINUX_SCHED_WAKE_Q_H
       3             : #define _LINUX_SCHED_WAKE_Q_H
       4             : 
       5             : /*
       6             :  * Wake-queues are lists of tasks with a pending wakeup, whose
       7             :  * callers have already marked the task as woken internally,
       8             :  * and can thus carry on. A common use case is being able to
       9             :  * do the wakeups once the corresponding user lock as been
      10             :  * released.
      11             :  *
      12             :  * We hold reference to each task in the list across the wakeup,
      13             :  * thus guaranteeing that the memory is still valid by the time
      14             :  * the actual wakeups are performed in wake_up_q().
      15             :  *
      16             :  * One per task suffices, because there's never a need for a task to be
      17             :  * in two wake queues simultaneously; it is forbidden to abandon a task
      18             :  * in a wake queue (a call to wake_up_q() _must_ follow), so if a task is
      19             :  * already in a wake queue, the wakeup will happen soon and the second
      20             :  * waker can just skip it.
      21             :  *
      22             :  * The DEFINE_WAKE_Q macro declares and initializes the list head.
      23             :  * wake_up_q() does NOT reinitialize the list; it's expected to be
      24             :  * called near the end of a function. Otherwise, the list can be
      25             :  * re-initialized for later re-use by wake_q_init().
      26             :  *
      27             :  * NOTE that this can cause spurious wakeups. schedule() callers
      28             :  * must ensure the call is done inside a loop, confirming that the
      29             :  * wakeup condition has in fact occurred.
      30             :  *
      31             :  * NOTE that there is no guarantee the wakeup will happen any later than the
      32             :  * wake_q_add() location. Therefore task must be ready to be woken at the
      33             :  * location of the wake_q_add().
      34             :  */
      35             : 
      36             : #include <linux/sched.h>
      37             : 
      38             : struct wake_q_head {
      39             :         struct wake_q_node *first;
      40             :         struct wake_q_node **lastp;
      41             : };
      42             : 
      43             : #define WAKE_Q_TAIL ((struct wake_q_node *) 0x01)
      44             : 
      45             : #define DEFINE_WAKE_Q(name)                             \
      46             :         struct wake_q_head name = { WAKE_Q_TAIL, &name.first }
      47             : 
      48           5 : static inline void wake_q_init(struct wake_q_head *head)
      49             : {
      50           5 :         head->first = WAKE_Q_TAIL;
      51           5 :         head->lastp = &head->first;
      52             : }
      53             : 
      54          18 : static inline bool wake_q_empty(struct wake_q_head *head)
      55             : {
      56          18 :         return head->first == WAKE_Q_TAIL;
      57             : }
      58             : 
      59             : extern void wake_q_add(struct wake_q_head *head, struct task_struct *task);
      60             : extern void wake_q_add_safe(struct wake_q_head *head, struct task_struct *task);
      61             : extern void wake_up_q(struct wake_q_head *head);
      62             : 
      63             : #endif /* _LINUX_SCHED_WAKE_Q_H */