fs/epoll: restore waking from ep_done_scan()

339ddb53d373 (fs/epoll: remove unnecessary wakeups of nested epoll)
changed the userspace visible behavior of exclusive waiters blocked on a
common epoll descriptor upon a single event becoming ready.  Previously,
all tasks doing epoll_wait would awake, and now only one is awoken,
potentially causing missed wakeups on applications that rely on this
behavior, such as Apache Qpid.

While the aforementioned commit aims at having only a wakeup single path
in ep_poll_callback (with the exceptions of epoll_ctl cases), we need to
restore the wakeup in what was the old ep_scan_ready_list() such that the
next thread can be awoken, in a cascading style, after the waker's
corresponding ep_send_events().

Link: https://lkml.kernel.org/r/20210405231025.33829-3-dave@stgolabs.net
Fixes: 339ddb53d373 ("fs/epoll: remove unnecessary wakeups of nested epoll")
Signed-off-by: Davidlohr Bueso <dbueso@suse.de>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Jason Baron <jbaron@akamai.com>
Cc: Roman Penyaev <rpenyaev@suse.de>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>

diff --git a/fs/eventpoll.c b/fs/eventpoll.c
index 73138ea6834252acce735346ff4235ae989525c5..1e596e1d0bba8c2fab0e82632c56aa7dd2386726 100644 (file)
--- a/fs/eventpoll.c
+++ b/fs/eventpoll.c
@@ -657,6 +657,12 @@ static void ep_done_scan(struct eventpoll *ep,
         */
        list_splice(txlist, &ep->rdllist);
        __pm_relax(ep->ws);
+
+       if (!list_empty(&ep->rdllist)) {
+               if (waitqueue_active(&ep->wq))
+                       wake_up(&ep->wq);
+       }
+
        write_unlock_irq(&ep->lock);
 }