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Subject: Re: [PATCH] epoll: prevent missed events on EPOLL_CTL_MOD
From: Eric Dumazet <eric.dumazet@gmail.com>
To: Eric Wong <normalperson@yhbt.net>
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Date: Wed, 02 Jan 2013 11:03:36 -0800
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On Wed, 2013-01-02 at 18:40 +0000, Eric Wong wrote:
> Eric Dumazet <eric.dumazet@gmail.com> wrote:
> > First, thanks for working on this issue.
> 
> No problem!
> 
> > It seems the real problem is the epi->event.events = event->events;
> > which is done without taking ep->lock
> 
> Yes.  I am hoping it is possible to do it without a lock there,
> but your change is more obviously correct.
> 
> > While a smp_mb() could reduce the race window, I believe there is still
> > a race, and the following patch would close it.
> 
> I'm not an experienced kernel hacker, can you describe where the race
> would be?

It would be for example in ep_send_events_proc() doing :

if (epi->event.events & EPOLLONESHOT)
    epi->event.events &= EP_PRIVATE_BITS;

And this could happen at the same time.