include/linux/rcuwait.h

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