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Date:   Wed, 12 Aug 2020 22:06:53 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Oleg Nesterov <oleg@redhat.com>
Cc:     Jens Axboe <axboe@kernel.dk>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Jann Horn <jannh@google.com>
Subject: Re: [PATCH] task_work: only grab task signal lock when needed
Message-ID: <20200812200653.GZ3982@worktop.programming.kicks-ass.net>
References: <0028d3ea-4d05-405f-b457-75c83d381d89@kernel.dk>
 <20200811152310.GF21797@redhat.com>
 <20200812145422.GA10232@redhat.com>
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On Wed, Aug 12, 2020 at 04:54:23PM +0200, Oleg Nesterov wrote:

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

Nope -- although I don't claim to understand the signal code much.

> 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();
> 

I agree this should work; smp_store_mb() isn't my favourite primitive,
but yes, this seems as good a use of it as there is so why not.