From mboxrd@z Thu Jan 1 00:00:00 1970 From: Paolo Bonzini Subject: Re: [PATCH 10/11] qspinlock: Paravirt support Date: Wed, 18 Jun 2014 14:03:12 +0200 Message-ID: <53A18000.8080909@redhat.com> References: <20140615124657.264658593@chello.nl> <20140615130154.213923590@chello.nl> <539F6AD5.4070301@hp.com> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii"; Format="flowed" Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <539F6AD5.4070301@hp.com> List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: virtualization-bounces@lists.linux-foundation.org Errors-To: virtualization-bounces@lists.linux-foundation.org To: Waiman Long , Peter Zijlstra Cc: linux-arch@vger.kernel.org, riel@redhat.com, gleb@redhat.com, kvm@vger.kernel.org, konrad.wilk@oracle.com, boris.ostrovsky@oracle.com, scott.norton@hp.com, raghavendra.kt@linux.vnet.ibm.com, paolo.bonzini@gmail.com, linux-kernel@vger.kernel.org, virtualization@lists.linux-foundation.org, Peter Zijlstra , chegu_vinod@hp.com, david.vrabel@citrix.com, oleg@redhat.com, xen-devel@lists.xenproject.org, tglx@linutronix.de, paulmck@linux.vnet.ibm.com, torvalds@linux-foundation.org, mingo@kernel.org List-Id: linux-arch.vger.kernel.org Il 17/06/2014 00:08, Waiman Long ha scritto: >> +void __pv_queue_unlock(struct qspinlock *lock) >> +{ >> + int val = atomic_read(&lock->val); >> + >> + native_queue_unlock(lock); >> + >> + if (val & _Q_LOCKED_SLOW) >> + ___pv_kick_head(lock); >> +} >> + > > Again a race can happen here between the reading and writing of the lock > value. I can't think of a good way to do that without using cmpxchg. Could you just use xchg on the locked byte? Paolo From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mx1.redhat.com ([209.132.183.28]:17100 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S965999AbaFRMER (ORCPT ); Wed, 18 Jun 2014 08:04:17 -0400 Message-ID: <53A18000.8080909@redhat.com> Date: Wed, 18 Jun 2014 14:03:12 +0200 From: Paolo Bonzini MIME-Version: 1.0 Subject: Re: [PATCH 10/11] qspinlock: Paravirt support References: <20140615124657.264658593@chello.nl> <20140615130154.213923590@chello.nl> <539F6AD5.4070301@hp.com> In-Reply-To: <539F6AD5.4070301@hp.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Sender: linux-arch-owner@vger.kernel.org List-ID: To: Waiman Long , Peter Zijlstra Cc: tglx@linutronix.de, mingo@kernel.org, linux-arch@vger.kernel.org, linux-kernel@vger.kernel.org, virtualization@lists.linux-foundation.org, xen-devel@lists.xenproject.org, kvm@vger.kernel.org, paolo.bonzini@gmail.com, konrad.wilk@oracle.com, boris.ostrovsky@oracle.com, paulmck@linux.vnet.ibm.com, riel@redhat.com, torvalds@linux-foundation.org, raghavendra.kt@linux.vnet.ibm.com, david.vrabel@citrix.com, oleg@redhat.com, gleb@redhat.com, scott.norton@hp.com, chegu_vinod@hp.com, Peter Zijlstra Message-ID: <20140618120312.7TMjwrBN5HemwCIzvy3ktwz3JctN90GeKKuIVG-m_qo@z> Il 17/06/2014 00:08, Waiman Long ha scritto: >> +void __pv_queue_unlock(struct qspinlock *lock) >> +{ >> + int val = atomic_read(&lock->val); >> + >> + native_queue_unlock(lock); >> + >> + if (val & _Q_LOCKED_SLOW) >> + ___pv_kick_head(lock); >> +} >> + > > Again a race can happen here between the reading and writing of the lock > value. I can't think of a good way to do that without using cmpxchg. Could you just use xchg on the locked byte? Paolo