linux-kernel - [PATCH v2 3/4] epoll: unify awaking of wakeup source on ep_poll

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Date:   Thu,  3 Jan 2019 16:01:03 +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 v2 3/4] epoll: unify awaking of wakeup source on ep_poll_callback() path

Original comment "Activate ep->ws since epi->ws may get deactivated
at any time" indeed sounds loud, but it is incorrect, because the path
where we check epi->ws is a path where insert to ovflist happens, i.e.
ep_scan_ready_list() has taken ep->mtx and waits for this callback
to finish, thus ep_modify() (which unregisters wakeup source) waits
for ep_scan_ready_list().

Here in this patch I simply call ep_pm_stay_awake_rcu(), which is
a bit extra for this path (indirectly protected by main ep->mtx, so
even rcu is not needed), but I do not want to create another naked
__ep_pm_stay_awake() variant only for this particular case, so rcu
variant is just better for all the cases.

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 | 9 +--------
 1 file changed, 1 insertion(+), 8 deletions(-)

diff --git a/fs/eventpoll.c b/fs/eventpoll.c
index 3c373a510f65..0122b9147542 100644
--- a/fs/eventpoll.c
+++ b/fs/eventpoll.c
@@ -1167,14 +1167,7 @@ static int ep_poll_callback(wait_queue_entry_t *wait, unsigned mode, int sync, v
 		if (epi->next == EP_UNACTIVE_PTR) {
 			epi->next = READ_ONCE(ep->ovflist);
 			WRITE_ONCE(ep->ovflist, epi);
-			if (epi->ws) {
-				/*
-				 * Activate ep->ws since epi->ws may get
-				 * deactivated at any time.
-				 */
-				__pm_stay_awake(ep->ws);
-			}
-
+			ep_pm_stay_awake_rcu(epi);
 		}
 		goto out_unlock;
 	}
-- 
2.19.1