+ fs-epoll-restore-waking-from-ep_done_scan.patch added to -mm tree

[akpm@linux-foundation.org]

The patch titled
     Subject: fs/epoll: restore waking from ep_done_scan()
has been added to the -mm tree.  Its filename is
     fs-epoll-restore-waking-from-ep_done_scan.patch

This patch should soon appear at
    https://ozlabs.org/~akpm/mmots/broken-out/fs-epoll-restore-waking-from-ep_done_scan.patch
and later at
    https://ozlabs.org/~akpm/mmotm/broken-out/fs-epoll-restore-waking-from-ep_done_scan.patch

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------------------------------------------------------
From: Davidlohr Bueso <dave@stgolabs.net>
Subject: 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
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>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 fs/eventpoll.c |    6 ++++++
 1 file changed, 6 insertions(+)

--- a/fs/eventpoll.c~fs-epoll-restore-waking-from-ep_done_scan
+++ a/fs/eventpoll.c
@@ -657,6 +657,12 @@ static void ep_done_scan(struct eventpol
 	 */
 	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);
 }
 
_

Patches currently in -mm which might be from dave@stgolabs.net are

kselftest-introduce-new-epoll-test-case.patch
fs-epoll-restore-waking-from-ep_done_scan.patch