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task_work: only grab task signal lock when needed 

If JOBCTL_TASK_WORK is already set on the targeted task, then we need
not go through {lock,unlock}_task_sighand() to set it again and queue
a signal wakeup. This is safe as we're checking it _after_ adding the
new task_work with cmpxchg().

The ordering is as follows:

task_work_add()				get_signal()
--------------------------------------------------------------
STORE(task->task_works, new_work);	STORE(task->jobctl);
mb();					mb();
LOAD(task->jobctl);			LOAD(task->task_works);

This speeds up TWA_SIGNAL handling quite a bit, which is important now
that io_uring is relying on it for all task_work deliveries.

Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Jann Horn <jannh@google.com>
Acked-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
This commit is contained in:
Jens Axboe 2020-08-13 09:01:38 -06:00
parent f254ac04c8
commit ebf0d100df
2 changed files with 22 additions and 2 deletions
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16
kernel/signal.c
View File

@ -2541,7 +2541,21 @@ bool get_signal(struct ksignal *ksig)
relock:
spin_lock_irq(&sighand->siglock);
current->jobctl &= ~JOBCTL_TASK_WORK;
/*
* Make sure we can safely read ->jobctl() in task_work add. As Oleg
* states:
*
* It pairs with mb (implied by cmpxchg) before READ_ONCE. So we
* roughly have
*
* task_work_add: get_signal:
* STORE(task->task_works, new_work); STORE(task->jobctl);
* mb(); mb();
* LOAD(task->jobctl); LOAD(task->task_works);
*
* and we can rely on STORE-MB-LOAD [ in task_work_add].
*/
smp_store_mb(current->jobctl, current->jobctl & ~JOBCTL_TASK_WORK);
if (unlikely(current->task_works)) {
spin_unlock_irq(&sighand->siglock);
task_work_run();

8
kernel/task_work.c
View File

@ -42,7 +42,13 @@ task_work_add(struct task_struct *task, struct callback_head *work, int notify)
set_notify_resume(task);
break;
case TWA_SIGNAL:
if (lock_task_sighand(task, &flags)) {
/*
* Only grab the sighand lock if we don't already have some
* task_work pending. This pairs with the smp_store_mb()
* in get_signal(), see comment there.
*/
if (!(READ_ONCE(task->jobctl) & JOBCTL_TASK_WORK) &&
lock_task_sighand(task, &flags)) {
task->jobctl |= JOBCTL_TASK_WORK;
signal_wake_up(task, 0);
unlock_task_sighand(task, &flags);