From: Roman Penyaev rpenyaev@suse.de Subject: 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 the a218cc491420 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.

Link: http://lkml.kernel.org/r/20200214170211.561524-1-rpenyaev@suse.de Fixes: a218cc491420 ("epoll: use rwlock in order to reduce ep_poll_callback() contention") References: https://bugzilla.kernel.org/show_bug.cgi?id=205933 Signed-off-by: Roman Penyaev rpenyaev@suse.de Reported-by: Max Neunhoeffer max@arangodb.com Bisected-by: Max Neunhoeffer max@arangodb.com 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+] Signed-off-by: Andrew Morton akpm@linux-foundation.org ---

fs/eventpoll.c | 8 ++++---- 1 file changed, 4 insertions(+), 4 deletions(-)

--- a/fs/eventpoll.c~epoll-fix-possible-lost-wakeup-on-epoll_ctl-path +++ a/fs/eventpoll.c @@ -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; _