From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-2.0 required=3.0 tests=DKIM_INVALID,DKIM_SIGNED, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, USER_AGENT_SANE_1 autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 3BA17C2D0DB for ; Thu, 23 Jan 2020 10:07:05 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 0C10F2467B for ; Thu, 23 Jan 2020 10:07:05 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=fail reason="signature verification failed" (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b="qf3j5dyR" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726771AbgAWKHE (ORCPT ); Thu, 23 Jan 2020 05:07:04 -0500 Received: from merlin.infradead.org ([205.233.59.134]:60824 "EHLO merlin.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726026AbgAWKHE (ORCPT ); Thu, 23 Jan 2020 05:07:04 -0500 DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=merlin.20170209; h=In-Reply-To:Content-Type:MIME-Version: References:Message-ID:Subject:Cc:To:From:Date:Sender:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description:Resent-Date: Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID:List-Id: List-Help:List-Unsubscribe:List-Subscribe:List-Post:List-Owner:List-Archive; bh=RNXTHzMWdAtx6D8MqtDjnWjGOecOvXnxd0b3aezRlbM=; b=qf3j5dyRRR2IDX21GjmYWm3EM gP7mN6IpY9OVHkaeBYNVonbidUDC8jdSaYepv223StC0EYxFYf8wIwQPwbPB8P3RHPnLgVIDpaq3H DiKB5VOUgs/ifvf6KqRW/fD9Ul8fWz8HhKfn03sIlXtEE+/3Dvj0gywlJvrBd63Dc3JaD+RXnHkUr +jN0JwEthW71geXwAsmHSD2c5s0pqueSyQd9StM8rWB+YwVZoyoNJKw1TOP1x6C93JSAGoRuSvZOX eFWaJWsTDzeN6ldkbGMpN29bDi7KoX6ZNoGicpVQJIRaOHXNQri1TCO2letCRjl8vKhuNdh3U3ycY P4JXp25aA==; Received: from j217100.upc-j.chello.nl ([24.132.217.100] helo=noisy.programming.kicks-ass.net) by merlin.infradead.org with esmtpsa (Exim 4.92.3 #3 (Red Hat Linux)) id 1iuZNo-0003Ff-11; Thu, 23 Jan 2020 10:06:40 +0000 Received: from hirez.programming.kicks-ass.net (hirez.programming.kicks-ass.net [192.168.1.225]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (Client did not present a certificate) by noisy.programming.kicks-ass.net (Postfix) with ESMTPS id E162F300B8D; Thu, 23 Jan 2020 11:04:55 +0100 (CET) Received: by hirez.programming.kicks-ass.net (Postfix, from userid 1000) id D52CE2B6E7A17; Thu, 23 Jan 2020 11:06:35 +0100 (CET) Date: Thu, 23 Jan 2020 11:06:35 +0100 From: Peter Zijlstra To: Alex Kogan Cc: linux@armlinux.org.uk, mingo@redhat.com, will.deacon@arm.com, arnd@arndb.de, longman@redhat.com, linux-arch@vger.kernel.org, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, tglx@linutronix.de, bp@alien8.de, hpa@zytor.com, x86@kernel.org, guohanjun@huawei.com, jglauber@marvell.com, steven.sistare@oracle.com, daniel.m.jordan@oracle.com, dave.dice@oracle.com Subject: Re: [PATCH v9 3/5] locking/qspinlock: Introduce CNA into the slow path of qspinlock Message-ID: <20200123100635.GE14946@hirez.programming.kicks-ass.net> References: <20200115035920.54451-1-alex.kogan@oracle.com> <20200115035920.54451-4-alex.kogan@oracle.com> <20200123092658.GC14879@hirez.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200123092658.GC14879@hirez.programming.kicks-ass.net> User-Agent: Mutt/1.10.1 (2018-07-13) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Jan 23, 2020 at 10:26:58AM +0100, Peter Zijlstra wrote: > On Tue, Jan 14, 2020 at 10:59:18PM -0500, Alex Kogan wrote: > > +/* this function is called only when the primary queue is empty */ > > +static inline bool cna_try_change_tail(struct qspinlock *lock, u32 val, > > + struct mcs_spinlock *node) > > +{ > > + struct mcs_spinlock *head_2nd, *tail_2nd; > > + u32 new; > > + > > + /* If the secondary queue is empty, do what MCS does. */ > > + if (node->locked <= 1) > > + return __try_clear_tail(lock, val, node); > > + > > + /* > > + * Try to update the tail value to the last node in the secondary queue. > > + * If successful, pass the lock to the first thread in the secondary > > + * queue. Doing those two actions effectively moves all nodes from the > > + * secondary queue into the main one. > > + */ > > + tail_2nd = decode_tail(node->locked); > > + head_2nd = tail_2nd->next; > > + new = ((struct cna_node *)tail_2nd)->encoded_tail + _Q_LOCKED_VAL; > > + > > + if (atomic_try_cmpxchg_relaxed(&lock->val, &val, new)) { > > + /* > > + * Try to reset @next in tail_2nd to NULL, but no need to check > > + * the result - if failed, a new successor has updated it. > > + */ > > I think you actually have an ordering bug here; the load of head_2nd > *must* happen before the atomic_try_cmpxchg(), otherwise it might > observe the new next and clear a valid next pointer. > > What would be the best fix for that; I'm thinking: > > head_2nd = smp_load_acquire(&tail_2nd->next); > > Will? Hmm, given we've not passed the lock around yet; why wouldn't something like this work: smp_store_release(&tail_2nd->next, NULL); if (!atomic_try_cmpxchg_relaxed(&lock, &val, new)) { tail_2nd->next = head_2nd; return false; } The whole second queue is only ever modified by the lock owner, and that is us, so we can pre-terminate the secondary queue (break the circular link), try the cmpxchg and fix it back up when it fails. > > + cmpxchg_relaxed(&tail_2nd->next, head_2nd, NULL); > > + arch_mcs_pass_lock(&head_2nd->locked, 1); > > + return true; > > + } > > + > > + return false; > > +} From mboxrd@z Thu Jan 1 00:00:00 1970 From: Peter Zijlstra Subject: Re: [PATCH v9 3/5] locking/qspinlock: Introduce CNA into the slow path of qspinlock Date: Thu, 23 Jan 2020 11:06:35 +0100 Message-ID: <20200123100635.GE14946@hirez.programming.kicks-ass.net> References: <20200115035920.54451-1-alex.kogan@oracle.com> <20200115035920.54451-4-alex.kogan@oracle.com> <20200123092658.GC14879@hirez.programming.kicks-ass.net> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Return-path: Content-Disposition: inline In-Reply-To: <20200123092658.GC14879@hirez.programming.kicks-ass.net> List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=m.gmane-mx.org@lists.infradead.org To: Alex Kogan Cc: linux-arch@vger.kernel.org, guohanjun@huawei.com, arnd@arndb.de, dave.dice@oracle.com, jglauber@marvell.com, x86@kernel.org, will.deacon@arm.com, linux@armlinux.org.uk, steven.sistare@oracle.com, linux-kernel@vger.kernel.org, mingo@redhat.com, bp@alien8.de, hpa@zytor.com, longman@redhat.com, tglx@linutronix.de, daniel.m.jordan@oracle.com, linux-arm-kernel@lists.infradead.org List-Id: linux-arch.vger.kernel.org On Thu, Jan 23, 2020 at 10:26:58AM +0100, Peter Zijlstra wrote: > On Tue, Jan 14, 2020 at 10:59:18PM -0500, Alex Kogan wrote: > > +/* this function is called only when the primary queue is empty */ > > +static inline bool cna_try_change_tail(struct qspinlock *lock, u32 val, > > + struct mcs_spinlock *node) > > +{ > > + struct mcs_spinlock *head_2nd, *tail_2nd; > > + u32 new; > > + > > + /* If the secondary queue is empty, do what MCS does. */ > > + if (node->locked <= 1) > > + return __try_clear_tail(lock, val, node); > > + > > + /* > > + * Try to update the tail value to the last node in the secondary queue. > > + * If successful, pass the lock to the first thread in the secondary > > + * queue. Doing those two actions effectively moves all nodes from the > > + * secondary queue into the main one. > > + */ > > + tail_2nd = decode_tail(node->locked); > > + head_2nd = tail_2nd->next; > > + new = ((struct cna_node *)tail_2nd)->encoded_tail + _Q_LOCKED_VAL; > > + > > + if (atomic_try_cmpxchg_relaxed(&lock->val, &val, new)) { > > + /* > > + * Try to reset @next in tail_2nd to NULL, but no need to check > > + * the result - if failed, a new successor has updated it. > > + */ > > I think you actually have an ordering bug here; the load of head_2nd > *must* happen before the atomic_try_cmpxchg(), otherwise it might > observe the new next and clear a valid next pointer. > > What would be the best fix for that; I'm thinking: > > head_2nd = smp_load_acquire(&tail_2nd->next); > > Will? Hmm, given we've not passed the lock around yet; why wouldn't something like this work: smp_store_release(&tail_2nd->next, NULL); if (!atomic_try_cmpxchg_relaxed(&lock, &val, new)) { tail_2nd->next = head_2nd; return false; } The whole second queue is only ever modified by the lock owner, and that is us, so we can pre-terminate the secondary queue (break the circular link), try the cmpxchg and fix it back up when it fails. > > + cmpxchg_relaxed(&tail_2nd->next, head_2nd, NULL); > > + arch_mcs_pass_lock(&head_2nd->locked, 1); > > + return true; > > + } > > + > > + return false; > > +} From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-2.3 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE, SPF_PASS,USER_AGENT_SANE_1 autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id E5C3AC2D0DB for ; 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Thu, 23 Jan 2020 10:06:40 +0000 Received: from hirez.programming.kicks-ass.net (hirez.programming.kicks-ass.net [192.168.1.225]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (Client did not present a certificate) by noisy.programming.kicks-ass.net (Postfix) with ESMTPS id E162F300B8D; Thu, 23 Jan 2020 11:04:55 +0100 (CET) Received: by hirez.programming.kicks-ass.net (Postfix, from userid 1000) id D52CE2B6E7A17; Thu, 23 Jan 2020 11:06:35 +0100 (CET) Date: Thu, 23 Jan 2020 11:06:35 +0100 From: Peter Zijlstra To: Alex Kogan Subject: Re: [PATCH v9 3/5] locking/qspinlock: Introduce CNA into the slow path of qspinlock Message-ID: <20200123100635.GE14946@hirez.programming.kicks-ass.net> References: <20200115035920.54451-1-alex.kogan@oracle.com> <20200115035920.54451-4-alex.kogan@oracle.com> <20200123092658.GC14879@hirez.programming.kicks-ass.net> MIME-Version: 1.0 Content-Disposition: inline In-Reply-To: <20200123092658.GC14879@hirez.programming.kicks-ass.net> User-Agent: Mutt/1.10.1 (2018-07-13) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: linux-arch@vger.kernel.org, guohanjun@huawei.com, arnd@arndb.de, dave.dice@oracle.com, jglauber@marvell.com, x86@kernel.org, will.deacon@arm.com, linux@armlinux.org.uk, steven.sistare@oracle.com, linux-kernel@vger.kernel.org, mingo@redhat.com, bp@alien8.de, hpa@zytor.com, longman@redhat.com, tglx@linutronix.de, daniel.m.jordan@oracle.com, linux-arm-kernel@lists.infradead.org Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+infradead-linux-arm-kernel=archiver.kernel.org@lists.infradead.org On Thu, Jan 23, 2020 at 10:26:58AM +0100, Peter Zijlstra wrote: > On Tue, Jan 14, 2020 at 10:59:18PM -0500, Alex Kogan wrote: > > +/* this function is called only when the primary queue is empty */ > > +static inline bool cna_try_change_tail(struct qspinlock *lock, u32 val, > > + struct mcs_spinlock *node) > > +{ > > + struct mcs_spinlock *head_2nd, *tail_2nd; > > + u32 new; > > + > > + /* If the secondary queue is empty, do what MCS does. */ > > + if (node->locked <= 1) > > + return __try_clear_tail(lock, val, node); > > + > > + /* > > + * Try to update the tail value to the last node in the secondary queue. > > + * If successful, pass the lock to the first thread in the secondary > > + * queue. Doing those two actions effectively moves all nodes from the > > + * secondary queue into the main one. > > + */ > > + tail_2nd = decode_tail(node->locked); > > + head_2nd = tail_2nd->next; > > + new = ((struct cna_node *)tail_2nd)->encoded_tail + _Q_LOCKED_VAL; > > + > > + if (atomic_try_cmpxchg_relaxed(&lock->val, &val, new)) { > > + /* > > + * Try to reset @next in tail_2nd to NULL, but no need to check > > + * the result - if failed, a new successor has updated it. > > + */ > > I think you actually have an ordering bug here; the load of head_2nd > *must* happen before the atomic_try_cmpxchg(), otherwise it might > observe the new next and clear a valid next pointer. > > What would be the best fix for that; I'm thinking: > > head_2nd = smp_load_acquire(&tail_2nd->next); > > Will? Hmm, given we've not passed the lock around yet; why wouldn't something like this work: smp_store_release(&tail_2nd->next, NULL); if (!atomic_try_cmpxchg_relaxed(&lock, &val, new)) { tail_2nd->next = head_2nd; return false; } The whole second queue is only ever modified by the lock owner, and that is us, so we can pre-terminate the secondary queue (break the circular link), try the cmpxchg and fix it back up when it fails. > > + cmpxchg_relaxed(&tail_2nd->next, head_2nd, NULL); > > + arch_mcs_pass_lock(&head_2nd->locked, 1); > > + return true; > > + } > > + > > + return false; > > +} _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel