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epoll: fix possible lost wakeup on epoll_ctl() path 

This fixes possible lost wakeup introduced by commit a218cc4914.
Originally modifications to ep->wq were serialized by ep->wq.lock, but
in commit a218cc4914 ("epoll: use rwlock in order to reduce
ep_poll_callback() contention") a new rw lock was introduced in order to
relax fd event path, i.e. callers of ep_poll_callback() function.

After the change ep_modify and ep_insert (both are called on epoll_ctl()
path) were switched to ep->lock, but ep_poll (epoll_wait) was using
ep->wq.lock on wqueue list modification.

The bug doesn't lead to any wqueue list corruptions, because wake up
path and list modifications were serialized by ep->wq.lock internally,
but actual waitqueue_active() check prior wake_up() call can be
reordered with modifications of ep ready list, thus wake up can be lost.

And yes, can be healed by explicit smp_mb():

  list_add_tail(&epi->rdlink, &ep->rdllist);
  smp_mb();
  if (waitqueue_active(&ep->wq))
	wake_up(&ep->wp);

But let's make it simple, thus current patch replaces ep->wq.lock with
the ep->lock for wqueue modifications, thus wake up path always observes
activeness of the wqueue correcty.

Fixes: a218cc4914 ("epoll: use rwlock in order to reduce ep_poll_callback() contention")
Reported-by: Max Neunhoeffer <max@arangodb.com>
Signed-off-by: Roman Penyaev <rpenyaev@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Tested-by: Max Neunhoeffer <max@arangodb.com>
Cc: Jakub Kicinski <kuba@kernel.org>
Cc: Christopher Kohlhoff <chris.kohlhoff@clearpool.io>
Cc: Davidlohr Bueso <dbueso@suse.de>
Cc: Jason Baron <jbaron@akamai.com>
Cc: Jes Sorensen <jes.sorensen@gmail.com>
Cc: <stable@vger.kernel.org>	[5.1+]
Link: http://lkml.kernel.org/r/20200214170211.561524-1-rpenyaev@suse.de
References: https://bugzilla.kernel.org/show_bug.cgi?id=205933
Bisected-by: Max Neunhoeffer <max@arangodb.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Roman Penyaev 2020-03-21 18:22:30 -07:00 committed by Linus Torvalds
parent 12e967fd8e
commit 1b53734bd0
1 changed files with 4 additions and 4 deletions
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8
fs/eventpoll.c
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@ -1854,9 +1854,9 @@ fetch_events:
waiter = true;
init_waitqueue_entry(&wait, current);
spin_lock_irq(&ep->wq.lock);
write_lock_irq(&ep->lock);
__add_wait_queue_exclusive(&ep->wq, &wait);
spin_unlock_irq(&ep->wq.lock);
write_unlock_irq(&ep->lock);
}
for (;;) {
@ -1904,9 +1904,9 @@ send_events:
goto fetch_events;
if (waiter) {
spin_lock_irq(&ep->wq.lock);
write_lock_irq(&ep->lock);
__remove_wait_queue(&ep->wq, &wait);
spin_unlock_irq(&ep->wq.lock);
write_unlock_irq(&ep->lock);
}
return res;