From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753341AbaJQQi6 (ORCPT ); Fri, 17 Oct 2014 12:38:58 -0400 Received: from foss-mx-na.foss.arm.com ([217.140.108.86]:45520 "EHLO foss-mx-na.foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752822AbaJQQi5 (ORCPT ); Fri, 17 Oct 2014 12:38:57 -0400 From: Catalin Marinas To: linux-kernel@vger.kernel.org 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 Date: Fri, 17 Oct 2014 17:38:49 +0100 Message-Id: <1413563929-2664-1-git-send-email-catalin.marinas@arm.com> X-Mailer: git-send-email 2.1.1 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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) case FUT_OFF_MMSHARED: futex_get_mm(key); /* implies MB (B) */ break; + default: + smp_mb(); /* explicit MB (B) */ } }