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Linux/kernel/task_work.c

  1 #include <linux/spinlock.h>
  2 #include <linux/task_work.h>
  3 #include <linux/tracehook.h>
  4 
  5 static struct callback_head work_exited; /* all we need is ->next == NULL */
  6 
  7 /**
  8  * task_work_add - ask the @task to execute @work->func()
  9  * @task: the task which should run the callback
 10  * @work: the callback to run
 11  * @notify: send the notification if true
 12  *
 13  * Queue @work for task_work_run() below and notify the @task if @notify.
 14  * Fails if the @task is exiting/exited and thus it can't process this @work.
 15  * Otherwise @work->func() will be called when the @task returns from kernel
 16  * mode or exits.
 17  *
 18  * This is like the signal handler which runs in kernel mode, but it doesn't
 19  * try to wake up the @task.
 20  *
 21  * Note: there is no ordering guarantee on works queued here.
 22  *
 23  * RETURNS:
 24  * 0 if succeeds or -ESRCH.
 25  */
 26 int
 27 task_work_add(struct task_struct *task, struct callback_head *work, bool notify)
 28 {
 29         struct callback_head *head;
 30 
 31         do {
 32                 head = READ_ONCE(task->task_works);
 33                 if (unlikely(head == &work_exited))
 34                         return -ESRCH;
 35                 work->next = head;
 36         } while (cmpxchg(&task->task_works, head, work) != head);
 37 
 38         if (notify)
 39                 set_notify_resume(task);
 40         return 0;
 41 }
 42 
 43 /**
 44  * task_work_cancel - cancel a pending work added by task_work_add()
 45  * @task: the task which should execute the work
 46  * @func: identifies the work to remove
 47  *
 48  * Find the last queued pending work with ->func == @func and remove
 49  * it from queue.
 50  *
 51  * RETURNS:
 52  * The found work or NULL if not found.
 53  */
 54 struct callback_head *
 55 task_work_cancel(struct task_struct *task, task_work_func_t func)
 56 {
 57         struct callback_head **pprev = &task->task_works;
 58         struct callback_head *work;
 59         unsigned long flags;
 60 
 61         if (likely(!task->task_works))
 62                 return NULL;
 63         /*
 64          * If cmpxchg() fails we continue without updating pprev.
 65          * Either we raced with task_work_add() which added the
 66          * new entry before this work, we will find it again. Or
 67          * we raced with task_work_run(), *pprev == NULL/exited.
 68          */
 69         raw_spin_lock_irqsave(&task->pi_lock, flags);
 70         while ((work = lockless_dereference(*pprev))) {
 71                 if (work->func != func)
 72                         pprev = &work->next;
 73                 else if (cmpxchg(pprev, work, work->next) == work)
 74                         break;
 75         }
 76         raw_spin_unlock_irqrestore(&task->pi_lock, flags);
 77 
 78         return work;
 79 }
 80 
 81 /**
 82  * task_work_run - execute the works added by task_work_add()
 83  *
 84  * Flush the pending works. Should be used by the core kernel code.
 85  * Called before the task returns to the user-mode or stops, or when
 86  * it exits. In the latter case task_work_add() can no longer add the
 87  * new work after task_work_run() returns.
 88  */
 89 void task_work_run(void)
 90 {
 91         struct task_struct *task = current;
 92         struct callback_head *work, *head, *next;
 93 
 94         for (;;) {
 95                 /*
 96                  * work->func() can do task_work_add(), do not set
 97                  * work_exited unless the list is empty.
 98                  */
 99                 do {
100                         work = READ_ONCE(task->task_works);
101                         head = !work && (task->flags & PF_EXITING) ?
102                                 &work_exited : NULL;
103                 } while (cmpxchg(&task->task_works, work, head) != work);
104 
105                 if (!work)
106                         break;
107                 /*
108                  * Synchronize with task_work_cancel(). It can't remove
109                  * the first entry == work, cmpxchg(task_works) should
110                  * fail, but it can play with *work and other entries.
111                  */
112                 raw_spin_unlock_wait(&task->pi_lock);
113 
114                 do {
115                         next = work->next;
116                         work->func(work);
117                         work = next;
118                         cond_resched();
119                 } while (work);
120         }
121 }
122 

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