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Date:   Fri, 14 Feb 2020 18:02:10 +0100
From:   Roman Penyaev <>
To:     unlisted-recipients:; (no To-header on input)
Cc:     Roman Penyaev <>,
        Max Neunhoeffer <>,
        Jakub Kicinski <>,
        Christopher Kohlhoff <>,
        Davidlohr Bueso <>,
        Jason Baron <>,
        Andrew Morton <>,,,
Subject: [PATCH v3 1/2] epoll: fix possible lost wakeup on epoll_ctl() path

This fixes possible lost wakeup introduced by the 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);
  if (waitqueue_active(&ep->wq))

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")
Signed-off-by: Roman Penyaev <>
Reported-by: Max Neunhoeffer <>
Bisected-by: Max Neunhoeffer <>
Tested-by: Max Neunhoeffer <>
Cc: Jakub Kicinski <>
Cc: Christopher Kohlhoff <>
Cc: Davidlohr Bueso <>
Cc: Jason Baron <>
Cc: Andrew Morton <>
Cc:  #5.1+
 Nothing was changed in v3
 Nothing interesting in v2:
     changed the comment a bit and specified Reported-by and Bisected-by tags

 fs/eventpoll.c | 8 ++++----
 1 file 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 @@ static int ep_poll(struct eventpoll *ep, struct epoll_event __user *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 @@ static int ep_poll(struct eventpoll *ep, struct epoll_event __user *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;

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