Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751632AbaBSA7M (ORCPT ); Tue, 18 Feb 2014 19:59:12 -0500 Received: from g4t3425.houston.hp.com ([15.201.208.53]:2001 "EHLO g4t3425.houston.hp.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750851AbaBSA7I (ORCPT ); Tue, 18 Feb 2014 19:59:08 -0500 Message-ID: <530401C9.4090100@hp.com> Date: Tue, 18 Feb 2014 19:58:49 -0500 From: Waiman Long User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:10.0.12) Gecko/20130109 Thunderbird/10.0.12 MIME-Version: 1.0 To: Peter Zijlstra CC: Thomas Gleixner , Ingo Molnar , "H. Peter Anvin" , Arnd Bergmann , linux-arch@vger.kernel.org, x86@kernel.org, linux-kernel@vger.kernel.org, Steven Rostedt , Andrew Morton , Michel Lespinasse , Andi Kleen , Rik van Riel , "Paul E. McKenney" , Linus Torvalds , Raghavendra K T , George Spelvin , Tim Chen , Daniel J Blueman , Alexander Fyodorov , Aswin Chandramouleeswaran , Scott J Norton , Thavatchai Makphaibulchoke Subject: Re: [PATCH v4 1/3] qspinlock: Introducing a 4-byte queue spinlock implementation References: <1392669684-4807-1-git-send-email-Waiman.Long@hp.com> <1392669684-4807-2-git-send-email-Waiman.Long@hp.com> <20140218073951.GZ27965@twins.programming.kicks-ass.net> <5303B6F3.9090001@hp.com> <20140218213748.GT14089@laptop.programming.kicks-ass.net> In-Reply-To: <20140218213748.GT14089@laptop.programming.kicks-ass.net> Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 02/18/2014 04:37 PM, Peter Zijlstra wrote: > On Tue, Feb 18, 2014 at 02:39:31PM -0500, Waiman Long wrote: >>>> + /* >>>> + * At the head of the wait queue now >>>> + */ >>>> + while (true) { >>>> + u32 qcode; >>>> + int retval; >>>> + >>>> + retval = queue_get_lock_qcode(lock,&qcode, my_qcode); >>>> + if (retval> 0) >>>> + ; /* Lock not available yet */ >>>> + else if (retval< 0) >>>> + /* Lock taken, can release the node& return */ >>>> + goto release_node; >>>> + else if (qcode != my_qcode) { >>>> + /* >>>> + * Just get the lock with other spinners waiting >>>> + * in the queue. >>>> + */ >>>> + if (queue_spin_trylock_unfair(lock)) >>>> + goto notify_next; >>> Why is this an option at all? >>> >>> >> Are you referring to the case (qcode != my_qcode)? This condition will be >> true if more than one tasks have queued up. > But in no case should we revert to unfair spinning or stealing. We > should always respect the queueing order. > > If the lock tail no longer points to us, then there's further waiters > and we should wait for ->next and unlock it -- after we've taken the > lock. > A task will be in this loop when it is already the head of a queue and is entitled to take the lock. The condition (qcode != my_qcode) is to decide whether it should just take the lock or take the lock & clear the code simultaneously. I am a bit cautious to use queue_spin_trylock_unfair() as there is a possibility that a CPU may run out of the queue node and need to do unfair busy spinning. -Longman -- 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/