netdev - Re: [PATCH] poll: prevent missed events if

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Date:	Tue, 1 Jan 2013 14:53:42 -0800
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Eric Wong <normalperson@...t.net>
Cc:	Eric Dumazet <eric.dumazet@...il.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Hans Verkuil <hans.verkuil@...co.com>,
	Jiri Olsa <jolsa@...hat.com>, Jonathan Corbet <corbet@....net>,
	Al Viro <viro@...iv.linux.org.uk>,
	Davide Libenzi <davidel@...ilserver.org>,
	Hans de Goede <hdegoede@...hat.com>,
	Mauro Carvalho Chehab <mchehab@...radead.org>,
	David Miller <davem@...emloft.net>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andreas Voellmy <andreas.voellmy@...e.edu>,
	"Junchang(Jason) Wang" <junchang.wang@...e.edu>,
	Network Development <netdev@...r.kernel.org>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>
Subject: Re: [PATCH] poll: prevent missed events if _qproc is NULL

On Tue, Jan 1, 2013 at 1:17 PM, Eric Wong <normalperson@...t.net> wrote:
>>
>> An alternate version (limited to epoll) would be:
>>
>> diff --git a/fs/eventpoll.c b/fs/eventpoll.c
>> index cd96649..ca5f3d0 100644
>> --- a/fs/eventpoll.c
>> +++ b/fs/eventpoll.c
>> @@ -1299,6 +1299,7 @@ static int ep_modify(struct eventpoll *ep, struct epitem *epi, struct epoll_even
>>        * Get current event bits. We can safely use the file* here because
>>        * its usage count has been increased by the caller of this function.
>>        */
>> +     smp_mb();
>>       revents = epi->ffd.file->f_op->poll(epi->ffd.file, &pt);
>>
>>       /*
>
> I was wrong, rereading 626cf236608505d376e4799adb4f7eb00a8594af,
> I think this race existed before.
>
> Perhaps my alternate patch above is a better fix.

Please document the barrier that this mb() pairs with, and then give
an explanation for the fix in the commit message, and I'll happily
take it. Even if it's just duplicating the comments above the
wq_has_sleeper() function, except modified for the ep_modify() case.

Of course, it would be good to get verification from Jason and Andreas
that the alternate patch also works for them.

               Linus
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