From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752477AbeC0KZc (ORCPT ); Tue, 27 Mar 2018 06:25:32 -0400 Received: from mail-wr0-f195.google.com ([209.85.128.195]:38392 "EHLO mail-wr0-f195.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752394AbeC0KZb (ORCPT ); Tue, 27 Mar 2018 06:25:31 -0400 X-Google-Smtp-Source: AG47ELuY6a5EYiEdpEt7s5D6AaKqCKwLZpb9SzdJWOzoyFVcZW67QDv18As9AinHFlozOTqw9BWYbg== Date: Tue, 27 Mar 2018 12:25:21 +0200 From: Andrea Parri To: Benjamin Herrenschmidt Cc: Paul Mackerras , Michael Ellerman , Peter Zijlstra , Ingo Molnar , linux-kernel@vger.kernel.org, Linus Torvalds Subject: Re: [PATCH for-4.17 2/2] powerpc: Remove smp_mb() from arch_spin_is_locked() Message-ID: <20180327102521.GA7347@andrea> References: <1522060667-7034-1-git-send-email-andrea.parri@amarulasolutions.com> <1522109216.7364.30.camel@kernel.crashing.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1522109216.7364.30.camel@kernel.crashing.org> 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 Tue, Mar 27, 2018 at 11:06:56AM +1100, Benjamin Herrenschmidt wrote: > On Mon, 2018-03-26 at 12:37 +0200, Andrea Parri wrote: > > Commit 51d7d5205d338 ("powerpc: Add smp_mb() to arch_spin_is_locked()") > > added an smp_mb() to arch_spin_is_locked(), in order to ensure that > > > > Thread 0 Thread 1 > > > > spin_lock(A); spin_lock(B); > > r0 = spin_is_locked(B) r1 = spin_is_locked(A); > > > > never ends up with r0 = r1 = 0, and reported one example (in ipc/sem.c) > > relying on such guarantee. > > > > It's however understood (and undocumented) that spin_is_locked() is not > > required to ensure such ordering guarantee, > > Shouldn't we start by documenting it ? I do sympathize with your concern about the documentation! ;) The patch in [1] was my (re)action to this concern; the sort of the patch is unclear to me by this time (and I'm not aware of other proposals in this respect). > > > guarantee that is currently > > _not_ provided by all implementations/arch, and that callers relying on > > such ordering should instead use suitable memory barriers before acting > > on the result of spin_is_locked(). > > > > Following a recent auditing[1] of the callers of {,raw_}spin_is_locked() > > revealing that none of them are relying on this guarantee anymore, this > > commit removes the leading smp_mb() from the primitive thus effectively > > reverting 51d7d5205d338. > > I would rather wait until it is properly documented. Debugging that IPC > problem took a *LOT* of time and energy, I wouldn't want these issues > to come and bite us again. I understand. And I'm grateful for this debugging as well as for the (IMO) excellent account of it you provided in 51d7d5205d338. Said this ;) I cannot except myself from saying that I would probably have resisted that solution (adding an smp_mb() in my arch_spin_is_locked), and instead "blamed"/suggested that caller to fix his memory ordering... Andrea > > > [1] https://marc.info/?l=linux-kernel&m=151981440005264&w=2 > > > > Signed-off-by: Andrea Parri > > Cc: Benjamin Herrenschmidt > > Cc: Paul Mackerras > > Cc: Michael Ellerman > > Cc: Peter Zijlstra > > Cc: Ingo Molnar > > Cc: Linus Torvalds > > --- > > arch/powerpc/include/asm/spinlock.h | 1 - > > 1 file changed, 1 deletion(-) > > > > diff --git a/arch/powerpc/include/asm/spinlock.h b/arch/powerpc/include/asm/spinlock.h > > index b9ebc3085fb79..ecc141e3f1a73 100644 > > --- a/arch/powerpc/include/asm/spinlock.h > > +++ b/arch/powerpc/include/asm/spinlock.h > > @@ -67,7 +67,6 @@ static __always_inline int arch_spin_value_unlocked(arch_spinlock_t lock) > > > > static inline int arch_spin_is_locked(arch_spinlock_t *lock) > > { > > - smp_mb(); > > return !arch_spin_value_unlocked(*lock); > > } > >