epoll: fix possible lost wakeup on epoll_ctl() path
This fixes possible lost wakeup introduced by commit a218cc491420.
Originally modifications to ep->wq were serialized by ep->wq.lock, but
in commit a218cc491420 ("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: a218cc491420 ("epoll: use rwlock in order to reduce ep_poll_callback() contention")
Reported-by: Max Neunhoeffer <[email protected]>
Signed-off-by: Roman Penyaev <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Tested-by: Max Neunhoeffer <[email protected]>
Cc: Jakub Kicinski <[email protected]>
Cc: Christopher Kohlhoff <[email protected]>
Cc: Davidlohr Bueso <[email protected]>
Cc: Jason Baron <[email protected]>
Cc: Jes Sorensen <[email protected]>
Cc: <[email protected]> [5.1+]
Link: http://lkml.kernel.org/r/[email protected]
References: https://bugzilla.kernel.org/show_bug.cgi?id=205933
Bisected-by: Max Neunhoeffer <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>
1 file changed
