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

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Message-ID: <20090626135742.GB3845@redhat.com>
Date:	Fri, 26 Jun 2009 15:57:42 +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 06/25, Davide Libenzi wrote:
>
> Not all the code that uses add_wait_queue() does need to have the MB,
> like code that does the most common pattern:
>
> xxx_poll(...) {
> 	poll_wait(...);
> 	lock();
> 	flags = calc_flags(->status);
> 	unlock();
> 	return flags;
> }
>
> xxx_update(...) {
> 	lock();
> 	->status = ...;
> 	unlock();
> 	if (waitqueue_active())
> 		wake_up();
> }
>
> It's the code that does the lockless flags calculation in ->poll that
> might need it.

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).

Oleg.

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