From: Roman Penyaev rpenyaev@suse.de
commit 1b53734bd0b2feed8e7761771b2e76fc9126ea0c upstream.
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 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 Bisected-by: Max Neunhoeffer max@arangodb.com Signed-off-by: Linus Torvalds torvalds@linux-foundation.org Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org
--- fs/eventpoll.c | 8 ++++---- 1 file changed, 4 insertions(+), 4 deletions(-)
--- a/fs/eventpoll.c +++ b/fs/eventpoll.c @@ -1881,9 +1881,9 @@ 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 (;;) { @@ -1931,9 +1931,9 @@ 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;