kernel/task_work.c

   1 // SPDX-License-Identifier: GPL-2.0
   2 #include <linux/spinlock.h>
   3 #include <linux/task_work.h>
   4 #include <linux/tracehook.h>
   5
   6 static struct callback_head work_exited; /* all we need is ->next == NULL */
   7
   8 /**
   9  * task_work_add - ask the @task to execute @work->func()
  10  * @task: the task which should run the callback
  11  * @work: the callback to run
  12  * @notify: how to notify the targeted task
  13  *
  14  * Queue @work for task_work_run() below and notify the @task if @notify
  15  * is @TWA_RESUME or @TWA_SIGNAL. @TWA_SIGNAL works like signals, in that the
  16  * it will interrupt the targeted task and run the task_work. @TWA_RESUME
  17  * work is run only when the task exits the kernel and returns to user mode,
  18  * or before entering guest mode. Fails if the @task is exiting/exited and thus
  19  * it can't process this @work. Otherwise @work->func() will be called when the
  20  * @task goes through one of the aforementioned transitions, or exits.
  21  *
  22  * If the targeted task is exiting, then an error is returned and the work item
  23  * is not queued. It's up to the caller to arrange for an alternative mechanism
  24  * in that case.
  25  *
  26  * Note: there is no ordering guarantee on works queued here. The task_work
  27  * list is LIFO.
  28  *
  29  * RETURNS:
  30  * 0 if succeeds or -ESRCH.
  31  */
  32 int task_work_add(struct task_struct *task, struct callback_head *work,
  33                   enum task_work_notify_mode notify)
  34 {
  35         struct callback_head *head;
  36
  37         do {
  38                 head = READ_ONCE(task->task_works);
  39                 if (unlikely(head == &work_exited))
  40                         return -ESRCH;
  41                 work->next = head;
  42         } while (cmpxchg(&task->task_works, head, work) != head);
  43
  44         switch (notify) {
  45         case TWA_NONE:
  46                 break;
  47         case TWA_RESUME:
  48                 set_notify_resume(task);
  49                 break;
  50         case TWA_SIGNAL:
  51                 set_notify_signal(task);
  52                 break;
  53         default:
  54                 WARN_ON_ONCE(1);
  55                 break;
  56         }
  57
  58         return 0;
  59 }
  60
  61 /**
  62  * task_work_cancel - cancel a pending work added by task_work_add()
  63  * @task: the task which should execute the work
  64  * @func: identifies the work to remove
  65  *
  66  * Find the last queued pending work with ->func == @func and remove
  67  * it from queue.
  68  *
  69  * RETURNS:
  70  * The found work or NULL if not found.
  71  */
  72 struct callback_head *
  73 task_work_cancel(struct task_struct *task, task_work_func_t func)
  74 {
  75         struct callback_head **pprev = &task->task_works;
  76         struct callback_head *work;
  77         unsigned long flags;
  78
  79         if (likely(!task->task_works))
  80                 return NULL;
  81         /*
  82          * If cmpxchg() fails we continue without updating pprev.
  83          * Either we raced with task_work_add() which added the
  84          * new entry before this work, we will find it again. Or
  85          * we raced with task_work_run(), *pprev == NULL/exited.
  86          */
  87         raw_spin_lock_irqsave(&task->pi_lock, flags);
  88         while ((work = READ_ONCE(*pprev))) {
  89                 if (work->func != func)
  90                         pprev = &work->next;
  91                 else if (cmpxchg(pprev, work, work->next) == work)
  92                         break;
  93         }
  94         raw_spin_unlock_irqrestore(&task->pi_lock, flags);
  95
  96         return work;
  97 }
  98
  99 /**
 100  * task_work_run - execute the works added by task_work_add()
 101  *
 102  * Flush the pending works. Should be used by the core kernel code.
 103  * Called before the task returns to the user-mode or stops, or when
 104  * it exits. In the latter case task_work_add() can no longer add the
 105  * new work after task_work_run() returns.
 106  */
 107 void task_work_run(void)
 108 {
 109         struct task_struct *task = current;
 110         struct callback_head *work, *head, *next;
 111
 112         for (;;) {
 113                 /*
 114                  * work->func() can do task_work_add(), do not set
 115                  * work_exited unless the list is empty.
 116                  */
 117                 do {
 118                         head = NULL;
 119                         work = READ_ONCE(task->task_works);
 120                         if (!work) {
 121                                 if (task->flags & PF_EXITING)
 122                                         head = &work_exited;
 123                                 else
 124                                         break;
 125                         }
 126                 } while (cmpxchg(&task->task_works, work, head) != work);
 127
 128                 if (!work)
 129                         break;
 130                 /*
 131                  * Synchronize with task_work_cancel(). It can not remove
 132                  * the first entry == work, cmpxchg(task_works) must fail.
 133                  * But it can remove another entry from the ->next list.
 134                  */
 135                 raw_spin_lock_irq(&task->pi_lock);
 136                 raw_spin_unlock_irq(&task->pi_lock);
 137
 138                 do {
 139                         next = work->next;
 140                         work->func(work);
 141                         work = next;
 142                         cond_resched();
 143                 } while (work);
 144         }
 145 }