Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote: > Please document the barrier that this mb() pairs with, and then give > an explanation for the fix in the commit message, and I'll happily > take it. Even if it's just duplicating the comments above the > wq_has_sleeper() function, except modified for the ep_modify() case. Hopefully my explanation is correct and makes sense below, I think both effects of the barrier are needed > Of course, it would be good to get verification from Jason and Andreas > that the alternate patch also works for them. Jason just confirmed it. ------------------------------- 8< ---------------------------- >From 02f43757d04bb6f2786e79eecf1cfa82e6574379 Mon Sep 17 00:00:00 2001 From: Eric Wong <normalperson@xxxxxxxx> Date: Tue, 1 Jan 2013 21:20:27 +0000 Subject: [PATCH] epoll: prevent missed events on EPOLL_CTL_MOD 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@xxxxxxxx> Cc: Hans Verkuil <hans.verkuil@xxxxxxxxx> Cc: Jiri Olsa <jolsa@xxxxxxxxxx> Cc: Jonathan Corbet <corbet@xxxxxxx> Cc: Al Viro <viro@xxxxxxxxxxxxxxxxxx> Cc: Davide Libenzi <davidel@xxxxxxxxxxxxxxx> Cc: Hans de Goede <hdegoede@xxxxxxxxxx> Cc: Mauro Carvalho Chehab <mchehab@xxxxxxxxxxxxx> Cc: David Miller <davem@xxxxxxxxxxxxx> Cc: Eric Dumazet <eric.dumazet@xxxxxxxxx> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> Cc: Andreas Voellmy <andreas.voellmy@xxxxxxxx> Tested-by: "Junchang(Jason) Wang" <junchang.wang@xxxxxxxx> Cc: netdev@xxxxxxxxxxxxxxx Cc: linux-fsdevel@xxxxxxxxxxxxxxx --- fs/eventpoll.c | 22 +++++++++++++++++++++- 1 file changed, 21 insertions(+), 1 deletion(-) diff --git a/fs/eventpoll.c b/fs/eventpoll.c index cd96649..39573ee 100644 --- a/fs/eventpoll.c +++ b/fs/eventpoll.c @@ -1285,7 +1285,7 @@ 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) { @@ -1296,6 +1296,26 @@ static int ep_modify(struct eventpoll *ep, struct epitem *epi, struct epoll_even } /* + * 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. */ -- Eric Wong -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
