[PATCH 3.16 073/127] futex: Ensure get_futex_key_refs() always implies a barrier

[Greg Kroah-Hartman]

3.16-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Catalin Marinas <catalin.marinas@xxxxxxx>

commit 76835b0ebf8a7fe85beb03c75121419a7dec52f0 upstream.

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 <catalin.marinas@xxxxxxx>
Reported-by: Matteo Franchin <Matteo.Franchin@xxxxxxx>
Fixes: b0c29f79ecea (futexes: Avoid taking the hb->lock if there's nothing to wake up)
Acked-by: Davidlohr Bueso <dave@xxxxxxxxxxxx>
Tested-by: Mike Galbraith <umgwanakikbuti@xxxxxxxxx>
Cc: Darren Hart <dvhart@xxxxxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

---
 kernel/futex.c |    2 ++
 1 file changed, 2 insertions(+)

--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -343,6 +343,8 @@ static void get_futex_key_refs(union fut
 	case FUT_OFF_MMSHARED:
 		futex_get_mm(key); /* implies MB (B) */
 		break;
+	default:
+		smp_mb(); /* explicit MB (B) */
 	}
 }
 


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