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Date:   Wed, 12 Aug 2020 16:54:23 +0200
From:   Oleg Nesterov <oleg@...hat.com>
To:     Jens Axboe <axboe@...nel.dk>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Jann Horn <jannh@...gle.com>
Subject: Re: [PATCH] task_work: only grab task signal lock when needed

On 08/11, Oleg Nesterov wrote:
>
> On 08/11, Jens Axboe wrote:
> >
> > --- a/kernel/task_work.c
> > +++ b/kernel/task_work.c
> > @@ -42,7 +42,8 @@ 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)) {
> > +		if (!(READ_ONCE(task->jobctl) & JOBCTL_TASK_WORK) &&
> > +		    lock_task_sighand(task, &flags)) {
>
> Aaaaah, sorry Jens, now I think this is racy. So I am glad I didn't add
> this optimization into the initial version ;)
>
> It is possible that JOBCTL_TASK_WORK is set but ->task_works == NULL. Say,
> task_work_add(TWA_SIGNAL) + task_work_cancel(), or the target task can call
> task_work_run() before it enters get_signal().
>
> And in this case another task_work_add(tsk, TWA_SIGNAL) can actually race
> with get_signal() which does
>
> 	current->jobctl &= ~JOBCTL_TASK_WORK;
> 	if (unlikely(current->task_works)) {
> 		spin_unlock_irq(&sighand->siglock);
> 		task_work_run();
>
> nothing guarantees that get_signal() sees ->task_works != NULL. Probably
> this is what Jann meant.
>
> We can probably add a barrier into get_signal() but I didn't sleep today,
> I'll try to think tomorrow.

I see nothing better than the additional change below. Peter, do you see
another solution?

This needs a comment to explain that this mb() pairs with another barrier
provided by cmpxchg() in task_work_add(). It ensures that either get_signal()
sees the new work added by task_work_add(), or task_work_add() sees the
result of "&= ~JOBCTL_TASK_WORK".

Oleg.

--- x/kernel/signal.c
+++ x/kernel/signal.c
@@ -2541,7 +2541,7 @@ bool get_signal(struct ksignal *ksig)
 
 relock:
 	spin_lock_irq(&sighand->siglock);
-	current->jobctl &= ~JOBCTL_TASK_WORK;
+	smp_store_mb(current->jobctl, current->jobctl & ~JOBCTL_TASK_WORK);
 	if (unlikely(current->task_works)) {
 		spin_unlock_irq(&sighand->siglock);
 		task_work_run();

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