linux-kernel - Re: [PATCH] poll: prevent missed events if

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Date:	Tue, 1 Jan 2013 18:21:03 -0500
From:	"Junchang(Jason) Wang" <junchang.wang@...e.edu>
To:	Linus Torvalds <torvalds@...ux-foundation.org>,
	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>,
	Network Development <netdev@...r.kernel.org>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>
Subject: Re: [PATCH] poll: prevent missed events if _qproc is NULL

Hi all,

The alternate patch from Eric works well too. Even though I didn't see
a performance boost compared with the old version, this one is clearer
to me. Thanks your guys.


Cheers!

--Jason


On Tue, Jan 1, 2013 at 5:53 PM, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
> 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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