From: Peter Zijlstra <peterz@infradead.org>
Date: Thu, 29 Nov 2018 13:44:49 +0000 (+0100)
Subject: futex: Fix (possible) missed wakeup
X-Git-Tag: v4.20.14~52
X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=8ed43fef1f983ec8dcc11c7feaa94104592caf33;p=thirdparty%2Fkernel%2Fstable.git

futex: Fix (possible) missed wakeup

[ 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>
---

diff --git a/kernel/futex.c b/kernel/futex.c
index 4d1b7db04e105..48449897476d5 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);
 }
 
 /*