Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755089AbaJXD1R (ORCPT ); Thu, 23 Oct 2014 23:27:17 -0400 Received: from homie.mail.dreamhost.com ([208.97.132.208]:53130 "EHLO homiemail-a61.g.dreamhost.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751507AbaJXD1P (ORCPT ); Thu, 23 Oct 2014 23:27:15 -0400 Message-ID: <1414121220.817.0.camel@linux-t7sj.site> Subject: [PATCH] futex: Mention key referencing differences between shared and private futexes 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" Date: Thu, 23 Oct 2014 20:27:00 -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 From: Davidlohr Bueso Update our documentation as of fix 76835b0ebf8 (futex: Ensure get_futex_key_refs() always implies a barrier). Explicitly state that we don't do key referencing for private futexes. Signed-off-by: Davidlohr Bueso --- kernel/futex.c | 14 ++++++++++---- 1 file changed, 10 insertions(+), 4 deletions(-) diff --git a/kernel/futex.c b/kernel/futex.c index f3a3a07..bbf071f 100644 --- a/kernel/futex.c +++ b/kernel/futex.c @@ -143,9 +143,8 @@ * * Where (A) orders the waiters increment and the futex value read through * atomic operations (see hb_waiters_inc) and where (B) orders the write - * to futex and the waiters read -- this is done by the barriers in - * get_futex_key_refs(), through either ihold or atomic_inc, depending on the - * futex type. + * to futex and the waiters read -- this is done by the barriers for both + * shared and private futexes in get_futex_key_refs(). * * This yields the following case (where X:=waiters, Y:=futex): * @@ -344,13 +343,20 @@ static void get_futex_key_refs(union futex_key *key) futex_get_mm(key); /* implies MB (B) */ break; default: + /* + * Private futexes do not hold reference on an inode or + * mm, therefore the only purpose of calling get_futex_key_refs + * is because we need the barrier for the lockless waiter check. + */ smp_mb(); /* explicit MB (B) */ } } /* * Drop a reference to the resource addressed by a key. - * The hash bucket spinlock must not be held. + * The hash bucket spinlock must not be held. This is + * a no-op for private futexes, see comment in the get + * counterpart. */ static void drop_futex_key_refs(union futex_key *key) { -- 1.8.4.5 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/