From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751606AbeDFTsN (ORCPT ); Fri, 6 Apr 2018 15:48:13 -0400 Received: from mail-wm0-f65.google.com ([74.125.82.65]:50681 "EHLO mail-wm0-f65.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751329AbeDFTsL (ORCPT ); Fri, 6 Apr 2018 15:48:11 -0400 X-Google-Smtp-Source: AIpwx49LafS4kL41jKSe6rbrW6BC9TspqOsQ3Yspc1sI4TO9kDTmuHd48vU8fs3oVbpvv4n8nF10Ng== From: Andrea Parri To: paulmck@linux.vnet.ibm.com Cc: linux-kernel@vger.kernel.org, Alan Stern , David Howells , Andrea Parri , Will Deacon , Peter Zijlstra , Boqun Feng , Nicholas Piggin , Jade Alglave , Luc Maranget , Akira Yokosawa , Ingo Molnar Subject: [PATCH v4 1/3] locking: Document the semantics of spin_is_locked() Date: Fri, 6 Apr 2018 21:47:39 +0200 Message-Id: <1523044060-5568-1-git-send-email-andrea.parri@amarulasolutions.com> X-Mailer: git-send-email 2.7.4 In-Reply-To: <1522600912-6208-2-git-send-email-andrea.parri@amarulasolutions.com> References: <1522600912-6208-2-git-send-email-andrea.parri@amarulasolutions.com> Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org There appeared to be a certain, recurrent uncertainty concerning the semantics of spin_is_locked(), likely a consequence of the fact that this semantics remains undocumented or that it has been historically linked to the (likewise unclear) semantics of spin_unlock_wait(). A recent auditing [1] of the callers of the primitive confirmed that none of them are relying on particular ordering guarantees; document this semantics by adding a docbook header to spin_is_locked(). Also, describe behaviors specific to certain CONFIG_SMP=n builds. [1] https://marc.info/?l=linux-kernel&m=151981440005264&w=2 https://marc.info/?l=linux-kernel&m=152042843808540&w=2 https://marc.info/?l=linux-kernel&m=152043346110262&w=2 Co-Developed-by: Andrea Parri Co-Developed-by: Alan Stern Co-Developed-by: David Howells Signed-off-by: Andrea Parri Signed-off-by: Alan Stern Signed-off-by: David Howells Cc: Will Deacon Cc: Peter Zijlstra Cc: Boqun Feng Cc: Nicholas Piggin Cc: Jade Alglave Cc: Luc Maranget Cc: "Paul E. McKenney" Cc: Akira Yokosawa Cc: Ingo Molnar --- include/linux/spinlock.h | 18 ++++++++++++++++++ 1 file changed, 18 insertions(+) diff --git a/include/linux/spinlock.h b/include/linux/spinlock.h index 4894d322d2584..1e8a464358384 100644 --- a/include/linux/spinlock.h +++ b/include/linux/spinlock.h @@ -380,6 +380,24 @@ static __always_inline int spin_trylock_irq(spinlock_t *lock) raw_spin_trylock_irqsave(spinlock_check(lock), flags); \ }) +/** + * spin_is_locked() - Check whether a spinlock is locked. + * @lock: Pointer to the spinlock. + * + * This function is NOT required to provide any memory ordering + * guarantees; it could be used for debugging purposes or, when + * additional synchronization is needed, accompanied with other + * constructs (memory barriers) enforcing the synchronization. + * + * Returns: 1 if @lock is locked, 0 otherwise. + * + * Note that the function only tells you that the spinlock is + * seen to be locked, not that it is locked on your CPU. + * + * Further, on CONFIG_SMP=n builds with CONFIG_DEBUG_SPINLOCK=n, + * the return value is always 0 (see include/linux/spinlock_up.h). + * Therefore you should not rely heavily on the return value. + */ static __always_inline int spin_is_locked(spinlock_t *lock) { return raw_spin_is_locked(&lock->rlock); -- 2.7.4