Re: [PATCH] poll: prevent missed events if _qproc is NULL

From: Junchang(Jason) Wang
Date: Tue Jan 01 2013 - 18:21:00 EST

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.



On Tue, Jan 1, 2013 at 5:53 PM, Linus Torvalds
<torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
> On Tue, Jan 1, 2013 at 1:17 PM, Eric Wong <normalperson@xxxxxxxx> 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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