From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751752AbeCZOOs (ORCPT ); Mon, 26 Mar 2018 10:14:48 -0400 Received: from mail-wr0-f195.google.com ([209.85.128.195]:46528 "EHLO mail-wr0-f195.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751866AbeCZOOp (ORCPT ); Mon, 26 Mar 2018 10:14:45 -0400 X-Google-Smtp-Source: AIpwx4+GdwwP9pRhkcpwx3I6pOAze/gDzN03tQvlu6j4cGN2pXdjXeHoUDbR4o79Fd+WgUpes/qLnQ== Date: Mon, 26 Mar 2018 16:14:36 +0200 From: Andrea Parri To: Will Deacon Cc: Catalin Marinas , Peter Zijlstra , Ingo Molnar , linux-kernel@vger.kernel.org, Linus Torvalds Subject: Re: [PATCH for-4.17 1/2] arm64: Remove smp_mb() from arch_spin_is_locked() Message-ID: <20180326141436.GA11739@andrea> References: <1522060641-6986-1-git-send-email-andrea.parri@amarulasolutions.com> <20180326105705.GA6579@arm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20180326105705.GA6579@arm.com> User-Agent: Mutt/1.5.24 (2015-08-30) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Mar 26, 2018 at 11:57:05AM +0100, Will Deacon wrote: > On Mon, Mar 26, 2018 at 12:37:21PM +0200, Andrea Parri wrote: > > Commit 38b850a73034f ("arm64: spinlock: order spin_{is_locked,unlock_wait} > > against local locks") added an smp_mb() to arch_spin_is_locked(), in order > > "to ensure that the lock value is always loaded after any other locks have > > been taken by the current CPU", and reported one example (the "insane case" > > in ipc/sem.c) relying on such guarantee. > > > > It is however understood (and not documented) that spin_is_locked() is not > > required to ensure such an ordering guarantee, guarantee that is currently > > _not_ provided by all implementations/architectures, and that callers rely- > > ing on such ordering should instead insert suitable memory barriers before > > acting on the result of spin_is_locked(). > > > > Following a recent auditing[1] of the callsites of {,raw_}spin_is_locked() > > revealing that none of these callers are relying on the ordering guarantee > > anymore, this commit removes the leading smp_mb() from this primitive thus > > effectively reverting 38b850a73034f. > > > > [1] https://marc.info/?l=linux-kernel&m=151981440005264&w=2 > > What is patch 2/2 in this series? I couldn't find it in the archive. 2/2 is this change for powerpc: https://marc.info/?l=linux-kernel&m=152206068707522&w=2 > > Assuming that patch doesn't do it, please can you remove the comment > about spin_is_locked from mutex_is_locked? I ended up with the patch below but I suspect that it's not what you had in mind; please let me know if you'd like me to add it into this series. > > Also -- does this mean we can kill the #ifndef queued_spin_is_locked > guards in asm-generic/qspinlock.h? I don't see why arch may want to override that definition (maybe lack of imagination?); please let me know if you'd like to see the #ifndef gone. Andrea --- >>From 1b77a9a70823620f1c98e43453edf5707d02074e Mon Sep 17 00:00:00 2001 From: Andrea Parri Date: Mon, 26 Mar 2018 15:03:58 +0200 Subject: [PATCH] mutex: Remove the comment about spin_is_locked() from mutex_is_locked() Still true/valid, but not particularly useful [IMO]. Suggested-by: Will Deacon Signed-off-by: Andrea Parri --- include/linux/mutex.h | 3 --- 1 file changed, 3 deletions(-) diff --git a/include/linux/mutex.h b/include/linux/mutex.h index cb3bbed4e6339..9921822c51585 100644 --- a/include/linux/mutex.h +++ b/include/linux/mutex.h @@ -147,9 +147,6 @@ extern void __mutex_init(struct mutex *lock, const char *name, */ static inline bool mutex_is_locked(struct mutex *lock) { - /* - * XXX think about spin_is_locked - */ return __mutex_owner(lock) != NULL; } -- 2.7.4 > > Will