[lkml]   [2020]   [Feb]   [3]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
Messages in this thread
Patch in this message
Subject[PATCH 1/3] 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 modification of ep ready list, thus
wake up can be lost.

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 <>
Cc: Max Neunhoeffer <>
Cc: Jakub Kicinski <>
Cc: Christopher Kohlhoff <>
Cc: Davidlohr Bueso <>
Cc: Jason Baron <>
Cc: Andrew Morton <>
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;
 \ /
  Last update: 2020-02-03 22:00    [W:0.084 / U:0.256 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site