From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757223Ab2IDOSR (ORCPT ); Tue, 4 Sep 2012 10:18:17 -0400 Received: from mx1.redhat.com ([209.132.183.28]:18358 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1757126Ab2IDOSP (ORCPT ); Tue, 4 Sep 2012 10:18:15 -0400 Date: Tue, 4 Sep 2012 17:19:24 +0300 From: "Michael S. Tsirkin" To: Paolo Bonzini Cc: "Nicholas A. Bellinger" , linux-kernel@vger.kernel.org, linux-scsi@vger.kernel.org, kvm@vger.kernel.org, rusty@rustcorp.com.au, jasowang@redhat.com, virtualization@lists.linux-foundation.org, Christoph Hellwig , Jens Axboe , target-devel Subject: Re: [PATCH 5/5] virtio-scsi: introduce multiqueue support Message-ID: <20120904141924.GK9805@redhat.com> References: <1346154857-12487-1-git-send-email-pbonzini@redhat.com> <1346154857-12487-6-git-send-email-pbonzini@redhat.com> <1346725294.4162.79.camel@haakon2.linux-iscsi.org> <5045A3B4.2030101@redhat.com> <20120904084628.GA8437@redhat.com> <5045D6FF.5020801@redhat.com> <20120904110905.GA9119@redhat.com> <5045E387.4030103@redhat.com> <20120904133543.GF9805@redhat.com> <50460615.3000006@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <50460615.3000006@redhat.com> Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Sep 04, 2012 at 03:45:57PM +0200, Paolo Bonzini wrote: > > Also - some kind of comment explaining why a similar race can not happen > > with this lock in place would be nice: I see why this specific race can > > not trigger but since lock is dropped later before you submit command, I > > have hard time convincing myself what exactly gurantees that vq is never > > switched before or even while command is submitted. > > Because tgt->reqs will never become zero (which is a necessary condition > for tgt->req_vq to change), as long as one request is executing > virtscsi_queuecommand. > > Paolo Yes but this logic would apparently imply the lock is not necessary, and it actually is. I am not saying anything is wrong just that it looks scary. -- MST From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Michael S. Tsirkin" Subject: Re: [PATCH 5/5] virtio-scsi: introduce multiqueue support Date: Tue, 4 Sep 2012 17:19:24 +0300 Message-ID: <20120904141924.GK9805@redhat.com> References: <1346154857-12487-1-git-send-email-pbonzini@redhat.com> <1346154857-12487-6-git-send-email-pbonzini@redhat.com> <1346725294.4162.79.camel@haakon2.linux-iscsi.org> <5045A3B4.2030101@redhat.com> <20120904084628.GA8437@redhat.com> <5045D6FF.5020801@redhat.com> <20120904110905.GA9119@redhat.com> <5045E387.4030103@redhat.com> <20120904133543.GF9805@redhat.com> <50460615.3000006@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Return-path: Content-Disposition: inline In-Reply-To: <50460615.3000006@redhat.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: Paolo Bonzini Cc: Jens Axboe , linux-scsi@vger.kernel.org, kvm@vger.kernel.org, linux-kernel@vger.kernel.org, virtualization@lists.linux-foundation.org, target-devel , Christoph Hellwig List-Id: linux-scsi@vger.kernel.org On Tue, Sep 04, 2012 at 03:45:57PM +0200, Paolo Bonzini wrote: > > Also - some kind of comment explaining why a similar race can not happen > > with this lock in place would be nice: I see why this specific race can > > not trigger but since lock is dropped later before you submit command, I > > have hard time convincing myself what exactly gurantees that vq is never > > switched before or even while command is submitted. > > Because tgt->reqs will never become zero (which is a necessary condition > for tgt->req_vq to change), as long as one request is executing > virtscsi_queuecommand. > > Paolo Yes but this logic would apparently imply the lock is not necessary, and it actually is. I am not saying anything is wrong just that it looks scary. -- MST