linux-kernel - [PATCH 1/3] epoll: make sure all elements in ready list are in FIFO order

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Date:   Wed, 12 Dec 2018 12:03:55 +0100
From:   Roman Penyaev <rpenyaev@...e.de>
To:     unlisted-recipients:; (no To-header on input)
Cc:     Roman Penyaev <rpenyaev@...e.de>, Davidlohr Bueso <dbueso@...e.de>,
        Jason Baron <jbaron@...mai.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: [PATCH 1/3] epoll: make sure all elements in ready list are in FIFO order

All coming events are stored in FIFO order and this is also should be
applicable to ->ovflist, which originally is stack, i.e. LIFO.

Thus to keep correct FIFO order ->ovflist should reversed by adding
elements to the head of the read list but not to the tail.

Signed-off-by: Roman Penyaev <rpenyaev@...e.de>
Cc: Davidlohr Bueso <dbueso@...e.de>
Cc: Jason Baron <jbaron@...mai.com>
Cc: Al Viro <viro@...iv.linux.org.uk>
Cc: Andrew Morton <akpm@...ux-foundation.org>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: linux-fsdevel@...r.kernel.org
Cc: linux-kernel@...r.kernel.org
---
 fs/eventpoll.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/fs/eventpoll.c b/fs/eventpoll.c
index 2329f96469e2..3627c2e07149 100644
--- a/fs/eventpoll.c
+++ b/fs/eventpoll.c
@@ -722,7 +722,11 @@ static __poll_t ep_scan_ready_list(struct eventpoll *ep,
 		 * contain them, and the list_splice() below takes care of them.
 		 */
 		if (!ep_is_linked(epi)) {
-			list_add_tail(&epi->rdllink, &ep->rdllist);
+			/*
+			 * ->ovflist is LIFO, so we have to reverse it in order
+			 * to keep in FIFO.
+			 */
+			list_add(&epi->rdllink, &ep->rdllist);
 			ep_pm_stay_awake(epi);
 		}
 	}
-- 
2.19.1