Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Date:   Thu, 13 Aug 2020 13:48:05 +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: <20200813114804.GA30049@redhat.com>
References: <0028d3ea-4d05-405f-b457-75c83d381d89@kernel.dk>
 <20200811152310.GF21797@redhat.com>
 <20200812145422.GA10232@redhat.com>
 <7c0eca6b-a9aa-4228-6abf-2eb4372f8fa7@kernel.dk>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <7c0eca6b-a9aa-4228-6abf-2eb4372f8fa7@kernel.dk>
User-Agent: Mutt/1.5.24 (2015-08-30)
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On 08/12, Jens Axboe wrote:
>
> On 8/12/20 8:54 AM, Oleg Nesterov wrote:
> >
> > --- 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 think this should work when paired with the READ_ONCE() on the
> task_work_add() side.

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.

> I haven't managed to reproduce badness with the
> existing one that doesn't have the smp_store_mb() here, so can't verify
> much beyond that...

Yes, the race is very unlikely. And the problem is minor, the target task
can miss the new work added by TWA_SIGNAL and return from get_signal() without
TIF_SIGPENDING.

> Are you going to send this out as a complete patch?

Jens, could you please send the patch? I am on vacation and travelling.
Feel free to add my ACK.

Oleg.