Re: [PATCH] epoll: prevent missed events on EPOLL_CTL_MOD

Eric Dumazet
2013-01-02T19:03:48Z

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.