The patch titled
Subject: epoll: fix possible lost wakeup on epoll_ctl() path
has been added to the -mm tree. Its filename is
epoll-fix-possible-lost-wakeup-on-epoll_ctl-path.patch
This patch should soon appear at
http://ozlabs.org/~akpm/mmots/broken-out/epoll-fix-possible-lost-wakeup-on-epoll_ctl-path.patch
and later at
http://ozlabs.org/~akpm/mmotm/broken-out/epoll-fix-possible-lost-wakeup-on-epoll_ctl-path.patch
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------------------------------------------------------
From: Roman Penyaev <rpenyaev@xxxxxxx>
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@xxxxxxx
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@xxxxxxx>
Reported-by: Max Neunhoeffer <max@xxxxxxxxxxxx>
Bisected-by: Max Neunhoeffer <max@xxxxxxxxxxxx>
Tested-by: Max Neunhoeffer <max@xxxxxxxxxxxx>
Cc: Jakub Kicinski <kuba@xxxxxxxxxx>
Cc: Christopher Kohlhoff <chris.kohlhoff@xxxxxxxxxxxx>
Cc: Davidlohr Bueso <dbueso@xxxxxxx>
Cc: Jason Baron <jbaron@xxxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx> [5.1+]
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
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;
_
Patches currently in -mm which might be from rpenyaev@xxxxxxx are
epoll-fix-possible-lost-wakeup-on-epoll_ctl-path.patch
kselftest-introduce-new-epoll-test-case.patch
