netdev - Re: [PATCH] net: fix race in the receive/select

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Date:	Fri, 26 Jun 2009 16:50:27 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Davide Libenzi <davidel@...ilserver.org>
Cc:	Eric Dumazet <eric.dumazet@...il.com>,
	Jiri Olsa <jolsa@...hat.com>, netdev@...r.kernel.org,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	fbl@...hat.com, nhorman@...hat.com, davem@...hat.com,
	Tejun Heo <htejun@...il.com>
Subject: Re: [PATCH] net: fix race in the receive/select

On 06/26, Davide Libenzi wrote:
>
> On Fri, 26 Jun 2009, Oleg Nesterov wrote:
>
> > And if we remove waitqueue_active() in xxx_update(), then lock/unlock is
> > not needed too.
> >
> > If xxx_poll() takes q->lock first, it can safely miss the changes in ->status
> > and schedule(): xxx_update() will take q->lock, notice the sleeper and wake
> > it up (ok, it will set ->triggered but this doesn't matter).
> >
> > If xxx_update() takes q->lock first, xxx_poll() must see the changes in
> > status after poll_wait()->unlock(&q->lock) (in fact, after lock, not unlock).
>
> Sure. The snippet above was just to show what typically the code does, not
> a suggestion on how to solve the socket case.

Yes, yes. I just meant you are right imho, we shouldn't add mb() into
add_wait_queue().

> But yeah, the problem in this case is the waitqueue_active() call. Without
> that, the wait queue lock/unlock in poll_wait() and the one in wake_up()
> guarantees the necessary barriers.
> Some might argue the costs of the lock/unlock of q->lock, and wonder if
> MBs are a more efficient solution. This is something I'm not going into.
> To me, it just looked not right having cross-matching MB in different
> subsystems.

This is subjective and thus up to maintainers, but personally I think you
are very, very right.

Perhaps we can add

	void sock_poll_wait(struct file *file, struct sock *sk, poll_table *pt)
	{
		if (pt) {
			poll_wait(file, sk->sk_sleep, pt);
			/*
			 * fat comment
			 */
			smp_mb(); // or smp_mb__after_unlock();
		}
	}

Oleg.

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