diff options
authorRoman Penyaev <>2020-03-21 18:22:30 -0700
committerLinus Torvalds <>2020-03-21 18:56:06 -0700
commit1b53734bd0b2feed8e7761771b2e76fc9126ea0c (patch)
parent12e967fd8e4e6c3d275b4c69c890adc838891300 (diff)
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 <> Signed-off-by: Roman Penyaev <> Signed-off-by: Andrew Morton <> Tested-by: Max Neunhoeffer <> Cc: Jakub Kicinski <> Cc: Christopher Kohlhoff <> Cc: Davidlohr Bueso <> Cc: Jason Baron <> Cc: Jes Sorensen <> Cc: <> [5.1+] Link: References: Bisected-by: Max Neunhoeffer <> Signed-off-by: Linus Torvalds <>
1 files changed, 4 insertions, 4 deletions
diff --git a/fs/eventpoll.c b/fs/eventpoll.c
index b041b66002db..eee3c92a9ebf 100644
--- a/fs/eventpoll.c
+++ b/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;