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Date:	Fri, 17 Oct 2014 17:38:49 +0100
From:	Catalin Marinas <>
Cc:	Matteo Franchin <>,
	Davidlohr Bueso <>,
	Linus Torvalds <>,
	Darren Hart <>,
	Thomas Gleixner <>,
	Peter Zijlstra <>,
	Ingo Molnar <>,
	"Paul E. McKenney" <>
Subject: [PATCH] futex: Ensure get_futex_key_refs() always implies a barrier

Commit b0c29f79ecea (futexes: Avoid taking the hb->lock if there's
nothing to wake up) changes the futex code to avoid taking a lock when
there are no waiters. This code has been subsequently fixed in commit
11d4616bd07f (futex: revert back to the explicit waiter counting code).
Both the original commit and the fix-up rely on get_futex_key_refs() to
always imply a barrier.

However, for private futexes, none of the cases in the switch statement
of get_futex_key_refs() would be hit and the function completes without
a memory barrier as required before checking the "waiters" in
futex_wake() -> hb_waiters_pending(). The consequence is a race with a
thread waiting on a futex on another CPU, allowing the waker thread to
read "waiters == 0" while the waiter thread to have read "futex_val ==
locked" (in kernel).

Without this fix, the problem (user space deadlocks) can be seen with
Android bionic's mutex implementation on an arm64 multi-cluster system.

Signed-off-by: Catalin Marinas <>
Reported-by: Matteo Franchin <>
Fixes: b0c29f79ecea (futexes: Avoid taking the hb->lock if there's nothing to wake up)
Cc: <>
Cc: Davidlohr Bueso <>
Cc: Linus Torvalds <>
Cc: Darren Hart <>
Cc: Thomas Gleixner <>
Cc: Peter Zijlstra <>
Cc: Ingo Molnar <>
Cc: Paul E. McKenney <>
 kernel/futex.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/kernel/futex.c b/kernel/futex.c
index 815d7af2ffe8..f3a3a071283c 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -343,6 +343,8 @@ static void get_futex_key_refs(union futex_key *key)
 		futex_get_mm(key); /* implies MB (B) */
+	default:
+		smp_mb(); /* explicit MB (B) */
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