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

From: Eric Wong <[email-masked]>
Date: Wed, 2 Jan 2013 18:40:10 +0000

Eric Dumazet <[email-masked]> 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?

> diff --git a/fs/eventpoll.c b/fs/eventpoll.c
> index be56b21..25e5c53 100644
> --- a/fs/eventpoll.c
> +++ b/fs/eventpoll.c
> @@ -1313,7 +1313,10 @@ static int ep_modify(struct eventpoll *ep, struct epitem *epi, struct epoll_even
>  	 * otherwise we might miss an event that happens between the
>  	 * f_op->poll() call and the new event set registering.
>  	 */
> +	spin_lock_irq(&ep->lock);
>  	epi->event.events = event->events;
> +	spin_unlock_irq(&ep->lock);
> +
>  	pt._key = event->events;
>  	epi->event.data = event->data; /* protected by mtx */
>  	if (epi->event.events & EPOLLWAKEUP) {