From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752275AbeC1JrS (ORCPT ); Wed, 28 Mar 2018 05:47:18 -0400 Received: from mail-wm0-f66.google.com ([74.125.82.66]:55957 "EHLO mail-wm0-f66.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751224AbeC1JrQ (ORCPT ); Wed, 28 Mar 2018 05:47:16 -0400 X-Google-Smtp-Source: AIpwx4/rWnXkyKL/0RimAbfuJWzRyMhVmp74bMaLyOMsiuuYturycyEBxZ9UOIf8skUWlBdym5IsuA== From: Andrea Parri To: Will Deacon , Catalin Marinas , Peter Zijlstra , Ingo Molnar Cc: Linus Torvalds , linux-kernel@vger.kernel.org, Andrea Parri Subject: [PATCH v2 for-4.17 1/3] arm64: Remove smp_mb() from arch_spin_is_locked() Date: Wed, 28 Mar 2018 11:46:59 +0200 Message-Id: <1522230419-12275-1-git-send-email-andrea.parri@amarulasolutions.com> X-Mailer: git-send-email 2.7.4 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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 Signed-off-by: Andrea Parri Acked-by: Will Deacon Cc: Will Deacon Cc: Catalin Marinas Cc: Peter Zijlstra Cc: Ingo Molnar Cc: Linus Torvalds --- arch/arm64/include/asm/spinlock.h | 5 ----- 1 file changed, 5 deletions(-) diff --git a/arch/arm64/include/asm/spinlock.h b/arch/arm64/include/asm/spinlock.h index ebdae15d665de..26c5bd7d88d8d 100644 --- a/arch/arm64/include/asm/spinlock.h +++ b/arch/arm64/include/asm/spinlock.h @@ -122,11 +122,6 @@ static inline int arch_spin_value_unlocked(arch_spinlock_t lock) static inline int arch_spin_is_locked(arch_spinlock_t *lock) { - /* - * Ensure prior spin_lock operations to other locks have completed - * on this CPU before we test whether "lock" is locked. - */ - smp_mb(); /* ^^^ */ return !arch_spin_value_unlocked(READ_ONCE(*lock)); } -- 2.7.4