From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752069AbeC0AH6 (ORCPT ); Mon, 26 Mar 2018 20:07:58 -0400 Received: from gate.crashing.org ([63.228.1.57]:34537 "EHLO gate.crashing.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751160AbeC0AH5 (ORCPT ); Mon, 26 Mar 2018 20:07:57 -0400 Message-ID: <1522109216.7364.30.camel@kernel.crashing.org> Subject: Re: [PATCH for-4.17 2/2] powerpc: Remove smp_mb() from arch_spin_is_locked() From: Benjamin Herrenschmidt To: Andrea Parri , Paul Mackerras , Michael Ellerman , Peter Zijlstra , Ingo Molnar Cc: linux-kernel@vger.kernel.org, Linus Torvalds Date: Tue, 27 Mar 2018 11:06:56 +1100 In-Reply-To: <1522060667-7034-1-git-send-email-andrea.parri@amarulasolutions.com> References: <1522060667-7034-1-git-send-email-andrea.parri@amarulasolutions.com> Content-Type: text/plain; charset="UTF-8" X-Mailer: Evolution 3.26.6 (3.26.6-1.fc27) 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 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 ? > 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. > [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); > } >