linux-kernel - [PATCH] eventpoll: fix missing wakeup for ovflist in ep_poll

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Message-Id: <20200424025057.118641-1-khazhy@google.com>
Date:   Thu, 23 Apr 2020 19:50:57 -0700
From:   Khazhismel Kumykov <khazhy@...gle.com>
To:     viro@...iv.linux.org.uk, rpenyaev@...e.de,
        akpm@...ux-foundation.org, r@....cc
Cc:     linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        Khazhismel Kumykov <khazhy@...gle.com>
Subject: [PATCH] eventpoll: fix missing wakeup for ovflist in ep_poll_callback

In the event that we add to ovflist, before 339ddb53d373 we would be
woken up by ep_scan_ready_list, and did no wakeup in ep_poll_callback.
With that wakeup removed, if we add to ovflist here, we may never wake
up. Rather than adding back the ep_scan_ready_list wakeup - which was
resulting un uncessary wakeups, trigger a wake-up in ep_poll_callback.

We noticed that one of our workloads was missing wakeups starting with
339ddb53d373 and upon manual inspection, this wakeup seemed missing to
me. With this patch added, we no longer see missing wakeups. I haven't
yet tried to make a small reproducer, but the existing kselftests in
filesystem/epoll passed for me with this patch.

Fixes: 339ddb53d373 ("fs/epoll: remove unnecessary wakeups of nested epoll")
Signed-off-by: Khazhismel Kumykov <khazhy@...gle.com>
---
 fs/eventpoll.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/fs/eventpoll.c b/fs/eventpoll.c
index 8c596641a72b..40cc89559cf6 100644
--- a/fs/eventpoll.c
+++ b/fs/eventpoll.c
@@ -1240,7 +1240,7 @@ static int ep_poll_callback(wait_queue_entry_t *wait, unsigned mode, int sync, v
 		if (epi->next == EP_UNACTIVE_PTR &&
 		    chain_epi_lockless(epi))
 			ep_pm_stay_awake_rcu(epi);
-		goto out_unlock;
+		goto out_wakeup_unlock;
 	}

 	/* If this file is already in the ready list we exit soon */
@@ -1249,6 +1249,7 @@ static int ep_poll_callback(wait_queue_entry_t *wait, unsigned mode, int sync, v
 		ep_pm_stay_awake_rcu(epi);
 	}

+out_wakeup_unlock:
 	/*
 	 * Wake up ( if active ) both the eventpoll wait list and the ->poll()
 	 * wait list.
-- 
2.26.2.303.gf8c07b1a785-goog