Commit 61e96496 authored by Oleg Nesterov's avatar Oleg Nesterov Committed by Linus Torvalds

task_work: use READ_ONCE/lockless_dereference, avoid pi_lock if !task_works

Change task_work_cancel() to use lockless_dereference(), this is what
the code really wants but we didn't have this helper when it was

Also add the fast-path task->task_works == NULL check, in the likely
case this task has no pending works and we can avoid

While at it, change other users of ACCESS_ONCE() to use READ_ONCE().

Link: default avatarOleg Nesterov <>
Cc: Andrea Parri <>
Cc: Peter Zijlstra <>
Signed-off-by: default avatarAndrew Morton <>
Signed-off-by: default avatarLinus Torvalds <>
parent 949bed2f
......@@ -29,7 +29,7 @@ task_work_add(struct task_struct *task, struct callback_head *work, bool notify)
struct callback_head *head;
do {
head = ACCESS_ONCE(task->task_works);
head = READ_ONCE(task->task_works);
if (unlikely(head == &work_exited))
return -ESRCH;
work->next = head;
......@@ -57,6 +57,9 @@ task_work_cancel(struct task_struct *task, task_work_func_t func)
struct callback_head **pprev = &task->task_works;
struct callback_head *work;
unsigned long flags;
if (likely(!task->task_works))
return NULL;
* If cmpxchg() fails we continue without updating pprev.
* Either we raced with task_work_add() which added the
......@@ -64,8 +67,7 @@ task_work_cancel(struct task_struct *task, task_work_func_t func)
* we raced with task_work_run(), *pprev == NULL/exited.
raw_spin_lock_irqsave(&task->pi_lock, flags);
while ((work = ACCESS_ONCE(*pprev))) {
while ((work = lockless_dereference(*pprev))) {
if (work->func != func)
pprev = &work->next;
else if (cmpxchg(pprev, work, work->next) == work)
......@@ -95,7 +97,7 @@ void task_work_run(void)
* work_exited unless the list is empty.
do {
work = ACCESS_ONCE(task->task_works);
work = READ_ONCE(task->task_works);
head = !work && (task->flags & PF_EXITING) ?
&work_exited : NULL;
} while (cmpxchg(&task->task_works, work, head) != work);
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