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

[Eric Dumazet]

On Wed, 2013-01-02 at 18:40 +0000, Eric Wong wrote:
> Eric Dumazet <eric.dumazet@xxxxxxxxx> 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.



--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/