include/linux/rcuwait.h

   1 /* SPDX-License-Identifier: GPL-2.0 */
   2 #ifndef _LINUX_RCUWAIT_H_
   3 #define _LINUX_RCUWAIT_H_
   4
   5 #include <linux/rcupdate.h>
   6
   7 /*
   8  * rcuwait provides a way of blocking and waking up a single
   9  * task in an rcu-safe manner; where it is forbidden to use
  10  * after exit_notify(). task_struct is not properly rcu protected,
  11  * unless dealing with rcu-aware lists, ie: find_task_by_*().
  12  *
  13  * Alternatively we have task_rcu_dereference(), but the return
  14  * semantics have different implications which would break the
  15  * wakeup side. The only time @task is non-nil is when a user is
  16  * blocked (or checking if it needs to) on a condition, and reset
  17  * as soon as we know that the condition has succeeded and are
  18  * awoken.
  19  */
  20 struct rcuwait {
  21         struct task_struct *task;
  22 };
  23
  24 #define __RCUWAIT_INITIALIZER(name)             \
  25         { .task = NULL, }
  26
  27 static inline void rcuwait_init(struct rcuwait *w)
  28 {
  29         w->task = NULL;
  30 }
  31
  32 extern void rcuwait_wake_up(struct rcuwait *w);
  33
  34 /*
  35  * The caller is responsible for locking around rcuwait_wait_event(),
  36  * such that writes to @task are properly serialized.
  37  */
  38 #define rcuwait_wait_event(w, condition)                                \
  39 ({                                                                      \
  40         /*                                                              \
  41          * Complain if we are called after do_exit()/exit_notify(),     \
  42          * as we cannot rely on the rcu critical region for the         \
  43          * wakeup side.                                                 \
  44          */                                                             \
  45         WARN_ON(current->exit_state);                                   \
  46                                                                         \
  47         rcu_assign_pointer((w)->task, current);                         \
  48         for (;;) {                                                      \
  49                 /*                                                      \
  50                  * Implicit barrier (A) pairs with (B) in               \
  51                  * rcuwait_wake_up().                                   \
  52                  */                                                     \
  53                 set_current_state(TASK_UNINTERRUPTIBLE);                \
  54                 if (condition)                                          \
  55                         break;                                          \
  56                                                                         \
  57                 schedule();                                             \
  58         }                                                               \
  59                                                                         \
  60         WRITE_ONCE((w)->task, NULL);                                    \
  61         __set_current_state(TASK_RUNNING);                              \
  62 })
  63
  64 #endif /* _LINUX_RCUWAIT_H_ */