From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pb0-f41.google.com (mail-pb0-f41.google.com [209.85.160.41]) by kanga.kvack.org (Postfix) with ESMTP id 7CF2B6B005C for ; Mon, 11 Nov 2013 21:09:56 -0500 (EST) Received: by mail-pb0-f41.google.com with SMTP id wy17so6110030pbc.14 for ; Mon, 11 Nov 2013 18:09:56 -0800 (PST) Received: from psmtp.com ([74.125.245.131]) by mx.google.com with SMTP id rr7si17603615pbc.225.2013.11.11.18.09.54 for ; Mon, 11 Nov 2013 18:09:55 -0800 (PST) Message-ID: <52818DDC.1020105@hp.com> Date: Mon, 11 Nov 2013 21:09:32 -0500 From: Waiman Long MIME-Version: 1.0 Subject: Re: [PATCH v5 4/4] MCS Lock: Barrier corrections References: <1383940358.11046.417.camel@schen9-DESK> <20131111181049.GL28302@mudshark.cambridge.arm.com> <1384204673.10046.6.camel@schen9-mobl3> In-Reply-To: <1384204673.10046.6.camel@schen9-mobl3> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: Tim Chen , Peter Zijlstra Cc: Will Deacon , Ingo Molnar , Andrew Morton , Thomas Gleixner , "linux-kernel@vger.kernel.org" , linux-mm , "linux-arch@vger.kernel.org" , Linus Torvalds , Andrea Arcangeli , Alex Shi , Andi Kleen , Michel Lespinasse , Davidlohr Bueso , Matthew R Wilcox , Dave Hansen , Rik van Riel , Peter Hurley , "Paul E.McKenney" , Raghavendra K T , George Spelvin , "H. Peter Anvin" , Arnd Bergmann , Aswin Chandramouleeswaran , Scott J Norton , "Figo.zhang" On 11/11/2013 04:17 PM, Tim Chen wrote: >> You could then augment that with [cmp]xchg_{acquire,release} as >> appropriate. >> >>> +/* >>> * In order to acquire the lock, the caller should declare a local node and >>> * pass a reference of the node to this function in addition to the lock. >>> * If the lock has already been acquired, then this will proceed to spin >>> @@ -37,15 +62,19 @@ void mcs_spin_lock(struct mcs_spinlock **lock, struct mcs_spinlock *node) >>> node->locked = 0; >>> node->next = NULL; >>> >>> - prev = xchg(lock, node); >>> + /* xchg() provides a memory barrier */ >>> + prev = xchg_acquire(lock, node); >>> if (likely(prev == NULL)) { >>> /* Lock acquired */ >>> return; >>> } >>> ACCESS_ONCE(prev->next) = node; >>> - smp_wmb(); >>> - /* Wait until the lock holder passes the lock down */ >>> - while (!ACCESS_ONCE(node->locked)) >>> + /* >>> + * Wait until the lock holder passes the lock down. >>> + * Using smp_load_acquire() provides a memory barrier that >>> + * ensures subsequent operations happen after the lock is acquired. >>> + */ >>> + while (!(smp_load_acquire(&node->locked))) >>> arch_mutex_cpu_relax(); > An alternate implementation is > while (!ACCESS_ONCE(node->locked)) > arch_mutex_cpu_relax(); > smp_load_acquire(&node->locked); > > Leaving the smp_load_acquire at the end to provide appropriate barrier. > Will that be acceptable? > > Tim I second Tim's opinion. It will be help to have a smp_mb_load_acquire() function that provide a memory barrier with load-acquire semantic. I don't think we need one for store-release as that will not be in a loop. Peter, what do you think about adding that to your patch? -Longman -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org