From patchwork Thu Apr 26 10:34:26 2018 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Will Deacon X-Patchwork-Id: 10365477 Return-Path: Received: from mail.wl.linuxfoundation.org (pdx-wl-mail.web.codeaurora.org [172.30.200.125]) by pdx-korg-patchwork.web.codeaurora.org (Postfix) with ESMTP id E4871601D3 for ; Thu, 26 Apr 2018 10:45:43 +0000 (UTC) Received: from mail.wl.linuxfoundation.org (localhost [127.0.0.1]) by mail.wl.linuxfoundation.org (Postfix) with ESMTP id D40B62909D for ; Thu, 26 Apr 2018 10:45:43 +0000 (UTC) Received: by mail.wl.linuxfoundation.org (Postfix, from userid 486) id C8DDA290A8; Thu, 26 Apr 2018 10:45:43 +0000 (UTC) X-Spam-Checker-Version: SpamAssassin 3.3.1 (2010-03-16) on pdx-wl-mail.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-2.9 required=2.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,MAILING_LIST_MULTI autolearn=ham version=3.3.1 Received: from bombadil.infradead.org (bombadil.infradead.org [198.137.202.133]) (using TLSv1.2 with cipher AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.wl.linuxfoundation.org (Postfix) with ESMTPS id 64FE72909D for ; Thu, 26 Apr 2018 10:45:43 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=lists.infradead.org; s=bombadil.20170209; h=Sender: Content-Transfer-Encoding:Content-Type:MIME-Version:Cc:List-Subscribe: List-Help:List-Post:List-Archive:List-Unsubscribe:List-Id:References: In-Reply-To:Message-Id:Date:Subject:To:From:Reply-To:Content-ID: Content-Description:Resent-Date:Resent-From:Resent-Sender:Resent-To:Resent-Cc :Resent-Message-ID:List-Owner; bh=V+VN8jSAzMElVi41Tg4SCnGbu0007MMiEZfdA1qAYE8=; b=ZEB7Xqt+6WT5iIGltvUyfI8xRu CYXtrFxAW7C11EFqkoQ23fV4aWLpsGKqTnEPpD+PO4TgB/9VGAGpdEwjonpdkSuFOxHvB3WfukhUT mV1kGsyWwXvfGT0cf6Y0ql2tPpKJEOdoPWmGjOVtvlQhSu0B5W3C7dQ4lsH6iXL8KjhOdifYKJXoM WK8VsTT3IRayFwYImOAD6T+O9cSPPxZZFfgYCX+G87SPWsWOcFo7YCQjRvQntoNWPHSkdw8SDNIwI P8PVS+vsguGmMAGuE3zGr2yNNkXsg7ll8QKeCNs5c5iTd04bSKLRbGI8AkrDGXZxYlInwOGnHjcS/ jRabLy7g==; Received: from localhost ([127.0.0.1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.90_1 #2 (Red Hat Linux)) id 1fBeP5-0001gp-Ll; Thu, 26 Apr 2018 10:45:31 +0000 Received: from foss.arm.com ([217.140.101.70]) by bombadil.infradead.org with esmtp (Exim 4.90_1 #2 (Red Hat Linux)) id 1fBeET-00022v-12 for linux-arm-kernel@lists.infradead.org; Thu, 26 Apr 2018 10:34:36 +0000 Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.72.51.249]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 890E919F6; Thu, 26 Apr 2018 03:34:10 -0700 (PDT) Received: from edgewater-inn.cambridge.arm.com (usa-sjc-imap-foss1.foss.arm.com [10.72.51.249]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPA id 5AF143F5B7; Thu, 26 Apr 2018 03:34:10 -0700 (PDT) Received: by edgewater-inn.cambridge.arm.com (Postfix, from userid 1000) id BD8361AE51DF; Thu, 26 Apr 2018 11:34:29 +0100 (BST) From: Will Deacon To: linux-kernel@vger.kernel.org Subject: [PATCH v3 12/14] locking/qspinlock: Use try_cmpxchg instead of cmpxchg when locking Date: Thu, 26 Apr 2018 11:34:26 +0100 Message-Id: <1524738868-31318-13-git-send-email-will.deacon@arm.com> X-Mailer: git-send-email 2.1.4 In-Reply-To: <1524738868-31318-1-git-send-email-will.deacon@arm.com> References: <1524738868-31318-1-git-send-email-will.deacon@arm.com> X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20180426_033433_166472_603C1A48 X-CRM114-Status: GOOD ( 13.88 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.21 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: peterz@infradead.org, boqun.feng@gmail.com, will.deacon@arm.com, longman@redhat.com, paulmck@linux.vnet.ibm.com, mingo@kernel.org, linux-arm-kernel@lists.infradead.org MIME-Version: 1.0 Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+patchwork-linux-arm=patchwork.kernel.org@lists.infradead.org X-Virus-Scanned: ClamAV using ClamSMTP When reaching the head of an uncontended queue on the qspinlock slow-path, using a try_cmpxchg instead of a cmpxchg operation to transition the lock work to _Q_LOCKED_VAL generates slightly better code for x86 and pretty much identical code for arm64. Cc: Ingo Molnar Reported-by: Peter Zijlstra Signed-off-by: Will Deacon --- kernel/locking/qspinlock.c | 19 +++++++++---------- 1 file changed, 9 insertions(+), 10 deletions(-) diff --git a/kernel/locking/qspinlock.c b/kernel/locking/qspinlock.c index fa5d2ab369f9..1e3ddc42135e 100644 --- a/kernel/locking/qspinlock.c +++ b/kernel/locking/qspinlock.c @@ -467,16 +467,15 @@ void queued_spin_lock_slowpath(struct qspinlock *lock, u32 val) * Otherwise, we only need to grab the lock. */ - /* In the PV case we might already have _Q_LOCKED_VAL set */ - if ((val & _Q_TAIL_MASK) == tail) { - /* - * The atomic_cond_read_acquire() call above has provided the - * necessary acquire semantics required for locking. - */ - old = atomic_cmpxchg_relaxed(&lock->val, val, _Q_LOCKED_VAL); - if (old == val) - goto release; /* No contention */ - } + /* + * In the PV case we might already have _Q_LOCKED_VAL set. + * + * The atomic_cond_read_acquire() call above has provided the + * necessary acquire semantics required for locking. + */ + if (((val & _Q_TAIL_MASK) == tail) && + atomic_try_cmpxchg_relaxed(&lock->val, &val, _Q_LOCKED_VAL)) + goto release; /* No contention */ /* Either somebody is queued behind us or _Q_PENDING_VAL is set */ set_locked(lock);