blob: 2cc3c810197574c03e19a90b0ceb6d6dc5ca170c [file] [log] [blame]
From 2aca31e23f90adcb6e54cff43260298d06c5ae0b Mon Sep 17 00:00:00 2001
From: Roman Penyaev <>
Date: Sat, 21 Mar 2020 18:22:30 -0700
Subject: [PATCH] epoll: fix possible lost wakeup on epoll_ctl() path
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);
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")
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+]
Bisected-by: Max Neunhoeffer <>
Signed-off-by: Linus Torvalds <>
Signed-off-by: Paul Gortmaker <>
diff --git a/fs/eventpoll.c b/fs/eventpoll.c
index 0f9c073d78d5..abe19510cd58 100644
--- a/fs/eventpoll.c
+++ b/fs/eventpoll.c
@@ -1881,9 +1881,9 @@ 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 (;;) {
@@ -1931,9 +1931,9 @@ 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;