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epoll: prevent missed events on EPOLL_CTL_MOD

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EPOLL_CTL_MOD sets the interest mask before calling f_op->poll() to
ensure events are not missed.  Since the modifications to the interest
mask are not protected by the same lock as ep_poll_callback, we need to
ensure the change is visible to other CPUs calling ep_poll_callback.

We also need to ensure f_op->poll() has an up-to-date view of past
events which occured before we modified the interest mask.  So this
barrier also pairs with the barrier in wq_has_sleeper().

This should guarantee either ep_poll_callback or f_op->poll() (or both)
will notice the readiness of a recently-ready/modified item.

This issue was encountered by Andreas Voellmy and Junchang(Jason) Wang in:
http://thread.gmane.org/gmane.linux.kernel/1408782/

Signed-off-by: Eric Wong <[email protected]>
Cc: Hans Verkuil <[email protected]>
Cc: Jiri Olsa <[email protected]>
Cc: Jonathan Corbet <[email protected]>
Cc: Al Viro <[email protected]>
Cc: Davide Libenzi <[email protected]>
Cc: Hans de Goede <[email protected]>
Cc: Mauro Carvalho Chehab <[email protected]>
Cc: David Miller <[email protected]>
Cc: Eric Dumazet <[email protected]>
Cc: Andrew Morton <[email protected]>
Cc: Andreas Voellmy <[email protected]>
Tested-by: "Junchang(Jason) Wang" <[email protected]>
Cc: [email protected]
Cc: [email protected]
Signed-off-by: Linus Torvalds <[email protected]>
This commit is contained in:
Eric Wong 2013-01-01 21:20:27 +00:00 • committed by Linus Torvalds
commit 128dd1759d
1 changed files with 21 additions and 1 deletions
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22
fs/eventpoll.c
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@ -1313,7 +1313,7 @@ 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.
*/
epi->event.events = event->events;
epi->event.events = event->events; /* need barrier below */
pt._key = event->events;
epi->event.data = event->data; /* protected by mtx */
if (epi->event.events & EPOLLWAKEUP) {
@ -1323,6 +1323,26 @@ static int ep_modify(struct eventpoll *ep, struct epitem *epi, struct epoll_even
ep_destroy_wakeup_source(epi);
}
/*
* The following barrier has two effects:
*
* 1) Flush epi changes above to other CPUs. This ensures
* we do not miss events from ep_poll_callback if an
* event occurs immediately after we call f_op->poll().
* We need this because we did not take ep->lock while
* changing epi above (but ep_poll_callback does take
* ep->lock).
*
* 2) We also need to ensure we do not miss _past_ events
* when calling f_op->poll(). This barrier also
* pairs with the barrier in wq_has_sleeper (see
* comments for wq_has_sleeper).
*
* This barrier will now guarantee ep_poll_callback or f_op->poll
* (or both) will notice the readiness of an item.
*/
smp_mb();
/*
* Get current event bits. We can safely use the file* here because
* its usage count has been increased by the caller of this function.