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From:   Sasha Levin <sashal@kernel.org>
To:     linux-kernel@vger.kernel.org, stable@vger.kernel.org
Cc:     Peter Zijlstra <peterz@infradead.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@kernel.org>, Sasha Levin <sashal@kernel.org>
Subject: [PATCH AUTOSEL 4.20 39/77] futex: Fix (possible) missed wakeup
Date:   Thu, 14 Feb 2019 21:08:17 -0500
Message-Id: <20190215020855.176727-39-sashal@kernel.org>
In-Reply-To: <20190215020855.176727-1-sashal@kernel.org>
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From: Peter Zijlstra <peterz@infradead.org>

[ Upstream commit b061c38bef43406df8e73c5be06cbfacad5ee6ad ]

We must not rely on wake_q_add() to delay the wakeup; in particular
commit:

  1d0dcb3ad9d3 ("futex: Implement lockless wakeups")

moved wake_q_add() before smp_store_release(&q->lock_ptr, NULL), which
could result in futex_wait() waking before observing ->lock_ptr ==
NULL and going back to sleep again.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Fixes: 1d0dcb3ad9d3 ("futex: Implement lockless wakeups")
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 kernel/futex.c | 13 ++++++++-----
 1 file changed, 8 insertions(+), 5 deletions(-)

diff --git a/kernel/futex.c b/kernel/futex.c
index 5cc8083a4c89..6189292244a3 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -1444,11 +1444,7 @@ static void mark_wake_futex(struct wake_q_head *wake_q, struct futex_q *q)
 	if (WARN(q->pi_state || q->rt_waiter, "refusing to wake PI futex\n"))
 		return;
 
-	/*
-	 * Queue the task for later wakeup for after we've released
-	 * the hb->lock. wake_q_add() grabs reference to p.
-	 */
-	wake_q_add(wake_q, p);
+	get_task_struct(p);
 	__unqueue_futex(q);
 	/*
 	 * The waiting task can free the futex_q as soon as q->lock_ptr = NULL
@@ -1458,6 +1454,13 @@ static void mark_wake_futex(struct wake_q_head *wake_q, struct futex_q *q)
 	 * plist_del in __unqueue_futex().
 	 */
 	smp_store_release(&q->lock_ptr, NULL);
+
+	/*
+	 * Queue the task for later wakeup for after we've released
+	 * the hb->lock. wake_q_add() grabs reference to p.
+	 */
+	wake_q_add(wake_q, p);
+	put_task_struct(p);
 }
 
 /*
-- 
2.19.1