Messages in this thread Patch in this message |  | | From | Roman Penyaev <> | Subject | [PATCH 1/3] epoll: fix possible lost wakeup on epoll_ctl() path | Date | Mon, 3 Feb 2020 21:59:05 +0100 |
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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
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