[PATCH 5.4 089/102] epoll: fix possible lost wakeup on epoll_ctl() path

[Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>]

From: Roman Penyaev <rpenyaev@xxxxxxx>

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);
  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.

Fixes: a218cc491420 ("epoll: use rwlock in order to reduce ep_poll_callback() contention")
Reported-by: Max Neunhoeffer <max@xxxxxxxxxxxx>
Signed-off-by: Roman Penyaev <rpenyaev@xxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
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: Jes Sorensen <jes.sorensen@xxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx>	[5.1+]
Link: http://lkml.kernel.org/r/20200214170211.561524-1-rpenyaev@xxxxxxx
Bisected-by: Max Neunhoeffer <max@xxxxxxxxxxxx>
Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

---
 fs/eventpoll.c |    8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

--- a/fs/eventpoll.c
+++ b/fs/eventpoll.c
@@ -1881,9 +1881,9 @@ 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 (;;) {
@@ -1931,9 +1931,9 @@ 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;