[PATCH 1/3] epoll: fix possible lost wakeup on epoll_ctl() path

From: Roman Penyaev <rpenyaev@suse.de>
To: unlisted-recipients:; (no To-header on input)
Cc: Roman Penyaev <rpenyaev@suse.de>,
	Max Neunhoeffer <max@arangodb.com>,
	Jakub Kicinski <kuba@kernel.org>,
	Christopher Kohlhoff <chris.kohlhoff@clearpool.io>,
	Davidlohr Bueso <dbueso@suse.de>, Jason Baron <jbaron@akamai.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: [PATCH 1/3] epoll: fix possible lost wakeup on epoll_ctl() path
Date: Mon,  3 Feb 2020 21:59:05 +0100	[thread overview]
Message-ID: <20200203205907.291929-1-rpenyaev@suse.de> (raw)

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")
References: https://bugzilla.kernel.org/show_bug.cgi?id=205933
Signed-off-by: Roman Penyaev <rpenyaev@suse.de>
Cc: Max Neunhoeffer <max@arangodb.com>
Cc: Jakub Kicinski <kuba@kernel.org>
Cc: Christopher Kohlhoff <chris.kohlhoff@clearpool.io>
Cc: Davidlohr Bueso <dbueso@suse.de>
Cc: Jason Baron <jbaron@akamai.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: linux-fsdevel@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
---
 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;
-- 
2.24.1

