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Date:   Tue, 11 Aug 2020 17:23:10 +0200
From:   Oleg Nesterov <oleg@redhat.com>
To:     Jens Axboe <axboe@kernel.dk>
Cc:     "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Jann Horn <jannh@google.com>
Subject: Re: [PATCH] task_work: only grab task signal lock when needed
Message-ID: <20200811152310.GF21797@redhat.com>
References: <0028d3ea-4d05-405f-b457-75c83d381d89@kernel.dk>
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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.

Oleg.