| From 8491f592b4d8b2f060e24846247a7e1f2405924b Mon Sep 17 00:00:00 2001 |
| From: Eric Wong <normalperson@yhbt.net> |
| Date: Tue, 1 Jan 2013 21:20:27 +0000 |
| Subject: [PATCH] epoll: prevent missed events on EPOLL_CTL_MOD |
| |
| commit 128dd1759d96ad36c379240f8b9463e8acfd37a1 upstream. |
| |
| 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 <normalperson@yhbt.net> |
| Cc: Hans Verkuil <hans.verkuil@cisco.com> |
| Cc: Jiri Olsa <jolsa@redhat.com> |
| Cc: Jonathan Corbet <corbet@lwn.net> |
| Cc: Al Viro <viro@zeniv.linux.org.uk> |
| Cc: Davide Libenzi <davidel@xmailserver.org> |
| Cc: Hans de Goede <hdegoede@redhat.com> |
| Cc: Mauro Carvalho Chehab <mchehab@infradead.org> |
| Cc: David Miller <davem@davemloft.net> |
| Cc: Eric Dumazet <eric.dumazet@gmail.com> |
| Cc: Andrew Morton <akpm@linux-foundation.org> |
| Cc: Andreas Voellmy <andreas.voellmy@yale.edu> |
| Tested-by: "Junchang(Jason) Wang" <junchang.wang@yale.edu> |
| Cc: netdev@vger.kernel.org |
| Cc: linux-fsdevel@vger.kernel.org |
| Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org> |
| Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com> |
| --- |
| fs/eventpoll.c | 22 +++++++++++++++++++++- |
| 1 file changed, 21 insertions(+), 1 deletion(-) |
| |
| diff --git a/fs/eventpoll.c b/fs/eventpoll.c |
| index 34ca5ca9c3e8..f8a6c0876a7a 100644 |
| --- a/fs/eventpoll.c |
| +++ b/fs/eventpoll.c |
| @@ -1033,10 +1033,30 @@ 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 */ |
| epi->event.data = event->data; /* protected by mtx */ |
| |
| /* |
| + * 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. |
| */ |
| -- |
| 1.8.5.2 |
| |