Commit 2368e6d3 authored Nov 29, 2018 by Peter Zijlstra Committed by Greg Kroah-Hartman Mar 05, 2019

futex: Fix (possible) missed wakeup



[ Upstream commit b061c38bef43406df8e73c5be06cbfacad5ee6ad ]

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

  1d0dcb3a ("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: 1d0dcb3a ("futex: Implement lockless wakeups")
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>

parent 653a1dbc

kernel/futex.c

+8 −5

Original line number	Diff line number	Diff line
		@@ -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);
		}

		/*