On Thu, 2013-01-03 at 00:55 +0000, Eric Wong wrote: > As far as I can tell, this bug affects every Linux 2.6+ release ever. > > Reduced Cc: from original commit, as this fixes a different bug from > what I intended to fix(!) and limited to epoll. Big thanks to > Andreas and Jason for bringing this to our attention and testing. > > ----------------------------- 8< ------------------------------ > From 128dd1759d96ad36c379240f8b9463e8acfd37a1 Mon Sep 17 00:00:00 2001 > From: Eric Wong <[email protected]> > Date: Tue, 1 Jan 2013 21:20:27 +0000 > Subject: [PATCH] epoll: prevent missed events on EPOLL_CTL_MOD > > commit 128dd1759d96ad36c379240f8b9463e8acfd37a1 upstream. [...]
Queued up for 3.2, thanks. I'm attaching the version I used.
Ben.
--
Ben Hutchings
Klipstein's 4th Law of Prototyping and Production:
A fail-safe circuit will destroy others.
From: Eric Wong <[email protected]> Date: Tue, 1 Jan 2013 21:20:27 +0000 Subject: 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 <[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]> [bwh: Backported to 3.2: adjust context] Signed-off-by: Ben Hutchings <[email protected]> --- fs/eventpoll.c | 22 +++++++++++++++++++++- 1 file changed, 21 insertions(+), 1 deletion(-) --- a/fs/eventpoll.c +++ b/fs/eventpoll.c @@ -1197,10 +1197,30 @@ static int ep_modify(struct eventpoll *e * 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. */
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