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

Eric Wong
2013-01-01T21:17:31Z

Eric Wong <normalperson@yhbt.net> wrote:
> Eric Dumazet <eric.dumazet@gmail.com> wrote:
> > commit 626cf236608505d376e4799adb4f7eb00a8594af should not have this
> > side effect, at least for poll()/select() functions. The epoll() changes
> > I am not yet very confident.
>
> I have a better explanation of the epoll problem below.
>
> 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 suspect a race already existed before this commit, it would be nice to
> > track it properly.
>
> I don't believe this race existed before that change.

I was wrong, rereading 626cf236608505d376e4799adb4f7eb00a8594af,
I think this race existed before.

Perhaps my alternate patch above is a better fix.