From mboxrd@z Thu Jan 1 00:00:00 1970 From: Tejun Heo Subject: Re: [PATCH RFC] virtio_net: fix refill related races Date: Tue, 20 Dec 2011 11:09:46 -0800 Message-ID: <20111220190946.GD10752@google.com> References: <20111207152120.GA23417@redhat.com> <8739cvisqe.fsf@rustcorp.com.au> <20111211144428.GB14381@redhat.com> <878vmioh10.fsf@rustcorp.com.au> <20111212115405.GB7946@redhat.com> <87iplltd0g.fsf@rustcorp.com.au> <20111220190908.GC25689@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: <20111220190908.GC25689@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: "Michael S. Tsirkin" Cc: Amit Shah , netdev@vger.kernel.org, linux-kernel@vger.kernel.org, virtualization@lists.linux-foundation.org List-Id: virtualization@lists.linuxfoundation.org Hello, Michael. On Tue, Dec 20, 2011 at 09:09:08PM +0200, Michael S. Tsirkin wrote: > Another question, wanted to make sure: > virtnet_poll does schedule_delayed_work(&vi->refill, 0); > separately refill work itself also does > schedule_delayed_work(&vi->refill, HZ/2); > If two such events happen twice, on different CPUs, we are still guaranteed > the work will only run once, right? No, it's not. Normal workqueues only guarantee non-reentrance on local CPU. If you want to guarantee that only one instance of a given item is executing across all CPUs, you need to use the nrt workqueue. Thanks. -- tejun From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752617Ab1LTTJ6 (ORCPT ); Tue, 20 Dec 2011 14:09:58 -0500 Received: from mail-iy0-f174.google.com ([209.85.210.174]:56371 "EHLO mail-iy0-f174.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751782Ab1LTTJv (ORCPT ); Tue, 20 Dec 2011 14:09:51 -0500 Date: Tue, 20 Dec 2011 11:09:46 -0800 From: Tejun Heo To: "Michael S. Tsirkin" Cc: Rusty Russell , Amit Shah , virtualization@lists.linux-foundation.org, netdev@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH RFC] virtio_net: fix refill related races Message-ID: <20111220190946.GD10752@google.com> References: <20111207152120.GA23417@redhat.com> <8739cvisqe.fsf@rustcorp.com.au> <20111211144428.GB14381@redhat.com> <878vmioh10.fsf@rustcorp.com.au> <20111212115405.GB7946@redhat.com> <87iplltd0g.fsf@rustcorp.com.au> <20111220190908.GC25689@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20111220190908.GC25689@redhat.com> User-Agent: Mutt/1.5.20 (2009-06-14) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hello, Michael. On Tue, Dec 20, 2011 at 09:09:08PM +0200, Michael S. Tsirkin wrote: > Another question, wanted to make sure: > virtnet_poll does schedule_delayed_work(&vi->refill, 0); > separately refill work itself also does > schedule_delayed_work(&vi->refill, HZ/2); > If two such events happen twice, on different CPUs, we are still guaranteed > the work will only run once, right? No, it's not. Normal workqueues only guarantee non-reentrance on local CPU. If you want to guarantee that only one instance of a given item is executing across all CPUs, you need to use the nrt workqueue. Thanks. -- tejun