From mboxrd@z Thu Jan 1 00:00:00 1970 From: Will Deacon Subject: Re: [PATCH v9 3/5] locking/qspinlock: Introduce CNA into the slow path of qspinlock Date: Thu, 23 Jan 2020 11:22:51 +0000 Message-ID: <20200123112251.GC18991@willie-the-truck> References: <20200115035920.54451-1-alex.kogan@oracle.com> <20200115035920.54451-4-alex.kogan@oracle.com> <20200123092658.GC14879@hirez.programming.kicks-ass.net> <20200123100635.GE14946@hirez.programming.kicks-ass.net> <20200123101649.GF14946@hirez.programming.kicks-ass.net> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Content-Disposition: inline In-Reply-To: <20200123101649.GF14946@hirez.programming.kicks-ass.net> Sender: linux-kernel-owner@vger.kernel.org To: Peter Zijlstra Cc: Alex Kogan , 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 11:16:49AM +0100, Peter Zijlstra wrote: > On Thu, Jan 23, 2020 at 11:06:35AM +0100, Peter Zijlstra wrote: > > 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); > > Argh, make that: > > tail_2nd->next = NULL; > > smp_wmb(); > > > if (!atomic_try_cmpxchg_relaxed(&lock, &val, new)) { ... or could you drop the smp_wmb() and make this atomic_try_cmpxchg_release()? To be honest, I've failed to understand the code prior to your changes in this area: it appears to reply on a control-dependency from the two cmpxchg_relaxed() calls (which isn't sufficient to order the store parts afaict) and I also don't get how we deal with a transiently circular primary queue. Will From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail.kernel.org ([198.145.29.99]:42800 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726771AbgAWLW6 (ORCPT ); Thu, 23 Jan 2020 06:22:58 -0500 Date: Thu, 23 Jan 2020 11:22:51 +0000 From: Will Deacon Subject: Re: [PATCH v9 3/5] locking/qspinlock: Introduce CNA into the slow path of qspinlock Message-ID: <20200123112251.GC18991@willie-the-truck> References: <20200115035920.54451-1-alex.kogan@oracle.com> <20200115035920.54451-4-alex.kogan@oracle.com> <20200123092658.GC14879@hirez.programming.kicks-ass.net> <20200123100635.GE14946@hirez.programming.kicks-ass.net> <20200123101649.GF14946@hirez.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200123101649.GF14946@hirez.programming.kicks-ass.net> Sender: linux-arch-owner@vger.kernel.org List-ID: To: Peter Zijlstra Cc: Alex Kogan , 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 Message-ID: <20200123112251.6bZgWJPraBt030DmeN-6dZUr6C1rAAStHSpuHGLtm_4@z> On Thu, Jan 23, 2020 at 11:16:49AM +0100, Peter Zijlstra wrote: > On Thu, Jan 23, 2020 at 11:06:35AM +0100, Peter Zijlstra wrote: > > 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); > > Argh, make that: > > tail_2nd->next = NULL; > > smp_wmb(); > > > if (!atomic_try_cmpxchg_relaxed(&lock, &val, new)) { ... or could you drop the smp_wmb() and make this atomic_try_cmpxchg_release()? To be honest, I've failed to understand the code prior to your changes in this area: it appears to reply on a control-dependency from the two cmpxchg_relaxed() calls (which isn't sufficient to order the store parts afaict) and I also don't get how we deal with a transiently circular primary queue. Will