From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751268AbaJRHdS (ORCPT ); Sat, 18 Oct 2014 03:33:18 -0400 Received: from homie.mail.dreamhost.com ([208.97.132.208]:47257 "EHLO homiemail-a8.g.dreamhost.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750860AbaJRHdQ (ORCPT ); Sat, 18 Oct 2014 03:33:16 -0400 Message-ID: <1413617580.29249.9.camel@linux-t7sj.site> Subject: Re: [PATCH] futex: Ensure get_futex_key_refs() always implies a barrier From: Davidlohr Bueso To: Catalin Marinas Cc: linux-kernel@vger.kernel.org, Matteo Franchin , Davidlohr Bueso , Linus Torvalds , Darren Hart , Thomas Gleixner , Peter Zijlstra , Ingo Molnar , "Paul E. McKenney" , Mike Galbraith Date: Sat, 18 Oct 2014 00:33:00 -0700 In-Reply-To: <1413563929-2664-1-git-send-email-catalin.marinas@arm.com> References: <1413563929-2664-1-git-send-email-catalin.marinas@arm.com> Content-Type: text/plain; charset="UTF-8" X-Mailer: Evolution 3.10.4 Mime-Version: 1.0 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, 2014-10-17 at 17:38 +0100, Catalin Marinas wrote: > 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(). Good catch, glad I ran into this thread (my email recently changed). Private process futex (PTHREAD_PROCESS_PRIVATE) have no reference on an inode or mm so it would need the explicit barrier in those cases. > 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). Yeah missing wakeups are a strong sign of a problem with the hb_waiters_pending() side. > 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) */ > } Should we comment that this default is for the private futex case? Otherwise: Acked-by: Davidlohr Bueso