Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751481Ab3I1EeK (ORCPT ); Sat, 28 Sep 2013 00:34:10 -0400 Received: from mail-bk0-f54.google.com ([209.85.214.54]:44298 "EHLO mail-bk0-f54.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750844Ab3I1EeI (ORCPT ); Sat, 28 Sep 2013 00:34:08 -0400 MIME-Version: 1.0 In-Reply-To: <20130928021947.GF9093@linux.vnet.ibm.com> References: <1380147049.3467.67.camel@schen9-DESK> <20130927152953.GA4464@linux.vnet.ibm.com> <1380310733.3467.118.camel@schen9-DESK> <20130927203858.GB9093@linux.vnet.ibm.com> <1380322005.3467.186.camel@schen9-DESK> <20130927230137.GE9093@linux.vnet.ibm.com> <20130928021947.GF9093@linux.vnet.ibm.com> Date: Fri, 27 Sep 2013 21:34:06 -0700 X-Google-Sender-Auth: NJyBNKeRNIBpQ85kaK9o3rAscyc Message-ID: Subject: Re: [PATCH v6 5/6] MCS Lock: Restructure the MCS lock defines and locking code into its own file From: Jason Low To: Paul McKenney Cc: Jason Low , Tim Chen , Waiman Long , Ingo Molnar , Andrew Morton , Andrea Arcangeli , Alex Shi , Andi Kleen , Michel Lespinasse , Davidlohr Bueso , Matthew R Wilcox , Dave Hansen , Peter Zijlstra , Rik van Riel , Peter Hurley , linux-kernel@vger.kernel.org, linux-mm Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2431 Lines: 69 On Fri, Sep 27, 2013 at 7:19 PM, Paul E. McKenney wrote: > On Fri, Sep 27, 2013 at 04:54:06PM -0700, Jason Low wrote: >> On Fri, Sep 27, 2013 at 4:01 PM, Paul E. McKenney >> wrote: >> > Yep. The previous lock holder's smp_wmb() won't keep either the compiler >> > or the CPU from reordering things for the new lock holder. They could for >> > example reorder the critical section to precede the node->locked check, >> > which would be very bad. >> >> Paul, Tim, Longman, >> >> How would you like the proposed changes below? > > Could you point me at what this applies to? I can find flaws looking > at random pieces, given a little luck, but at some point I need to look > at the whole thing. ;-) Sure. Here is a link to the patch we are trying to modify: https://lkml.org/lkml/2013/9/25/532 Also, below is what the mcs_spin_lock() and mcs_spin_unlock() functions would look like after applying the proposed changes. static noinline void mcs_spin_lock(struct mcs_spin_node **lock, struct mcs_spin_node *node) { struct mcs_spin_node *prev; /* Init node */ node->locked = 0; node->next = NULL; prev = xchg(lock, node); if (likely(prev == NULL)) { /* Lock acquired. No need to set node->locked since it won't be used */ return; } ACCESS_ONCE(prev->next) = node; /* Wait until the lock holder passes the lock down */ while (!ACCESS_ONCE(node->locked)) arch_mutex_cpu_relax(); smp_mb(); } static void mcs_spin_unlock(struct mcs_spin_node **lock, struct mcs_spin_node *node) { struct mcs_spin_node *next = ACCESS_ONCE(node->next); if (likely(!next)) { /* * Release the lock by setting it to NULL */ if (cmpxchg(lock, node, NULL) == node) return; /* Wait until the next pointer is set */ while (!(next = ACCESS_ONCE(node->next))) arch_mutex_cpu_relax(); } smp_wmb(); ACCESS_ONCE(next->locked) = 1; } -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/