From: Oleg Nesterov <oleg@xxxxxxxxxx> Subject: 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 written. Also add the fast-path task->task_works == NULL check, in the likely case this task has no pending works and we can avoid spin_lock(task->pi_lock). While at it, change other users of ACCESS_ONCE() to use READ_ONCE(). Link: http://lkml.kernel.org/r/20160610150042.GA13868@xxxxxxxxxx Signed-off-by: Oleg Nesterov <oleg@xxxxxxxxxx> Cc: Andrea Parri <parri.andrea@xxxxxxxxx> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> --- kernel/task_work.c | 10 ++++++---- 1 file changed, 6 insertions(+), 4 deletions(-) diff -puN kernel/task_work.c~task_work-use-read_once-lockless_dereference-avoid-pi_lock-if-task_works kernel/task_work.c --- a/kernel/task_work.c~task_work-use-read_once-lockless_dereference-avoid-pi_lock-if-task_works +++ a/kernel/task_work.c @@ -29,7 +29,7 @@ task_work_add(struct task_struct *task, 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 *tas 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 *tas * we raced with task_work_run(), *pprev == NULL/exited. */ raw_spin_lock_irqsave(&task->pi_lock, flags); - while ((work = ACCESS_ONCE(*pprev))) { - smp_read_barrier_depends(); + 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); _ -- To unsubscribe from this list: send the line "unsubscribe mm-commits" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
