Re: [PATCH] epoll: prevent missed events on EPOLL_CTL_MOD
From: Eric Wong <hidden>
Date: 2013-01-02 18:40:11
Also in:
linux-fsdevel, lkml
Eric Dumazet [off-list ref] 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?
quoted hunk ↗ jump to hunk
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) {