From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S935410AbeBMNaP (ORCPT ); Tue, 13 Feb 2018 08:30:15 -0500 Received: from usa-sjc-mx-foss1.foss.arm.com ([217.140.101.70]:57618 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S935174AbeBMNaO (ORCPT ); Tue, 13 Feb 2018 08:30:14 -0500 From: Will Deacon To: linux-kernel@vger.kernel.org Cc: Will Deacon , Peter Zijlstra , "Paul E. McKenney" Subject: [PATCH] atomic/bitops: Clarify ordering semantics for failed test_and_{}_bit() Date: Tue, 13 Feb 2018 13:30:19 +0000 Message-Id: <1518528619-20049-1-git-send-email-will.deacon@arm.com> X-Mailer: git-send-email 2.1.4 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org A test_and_{}_bit() operation fails if the value of the bit is such that the modification does not take place. For example, if test_and_set_bit() returns 1. In these cases, follow the behaviour of cmpxchg and allow the operation to be unordered. This also applies to test_and_set_bit_lock() if the lock is found to be be taken already. Cc: Peter Zijlstra Cc: "Paul E. McKenney" Signed-off-by: Will Deacon --- Documentation/atomic_bitops.txt | 7 ++++++- include/asm-generic/bitops/lock.h | 3 ++- 2 files changed, 8 insertions(+), 2 deletions(-) diff --git a/Documentation/atomic_bitops.txt b/Documentation/atomic_bitops.txt index 5550bfdcce5f..be70b32c95d9 100644 --- a/Documentation/atomic_bitops.txt +++ b/Documentation/atomic_bitops.txt @@ -58,7 +58,12 @@ Like with atomic_t, the rule of thumb is: - RMW operations that have a return value are fully ordered. -Except for test_and_set_bit_lock() which has ACQUIRE semantics and + - RMW operations that are conditional are unordered on FAILURE, + otherwise the above rules apply. In the case of test_and_{}_bit() operations, + if the bit in memory is unchanged by the operation then it is deemed to have + failed. + +Except for a successful test_and_set_bit_lock() which has ACQUIRE semantics and clear_bit_unlock() which has RELEASE semantics. Since a platform only has a single means of achieving atomic operations diff --git a/include/asm-generic/bitops/lock.h b/include/asm-generic/bitops/lock.h index bc397573c43a..67ab280ad134 100644 --- a/include/asm-generic/bitops/lock.h +++ b/include/asm-generic/bitops/lock.h @@ -7,7 +7,8 @@ * @nr: Bit to set * @addr: Address to count from * - * This operation is atomic and provides acquire barrier semantics. + * This operation is atomic and provides acquire barrier semantics if + * the returned value is 0. * It can be used to implement bit locks. */ #define test_and_set_bit_lock(nr, addr) test_and_set_bit(nr, addr) -- 2.1.4