From mboxrd@z Thu Jan 1 00:00:00 1970 From: Eric Dumazet Subject: Re: [PATCH] virtio-net: fix a race on 32bit arches Date: Wed, 06 Jun 2012 22:06:11 +0200 Message-ID: <1339013171.26966.60.camel@edumazet-glaptop> References: <1338971724.2760.3913.camel@edumazet-glaptop> <1338972341.2760.3944.camel@edumazet-glaptop> <20120606111357.GA15070@redhat.com> <1338988210.2760.4485.camel@edumazet-glaptop> <20120606144941.GA17092@redhat.com> <1338995944.26966.6.camel@edumazet-glaptop> <20120606161715.GA17575@redhat.com> <1339002782.26966.22.camel@edumazet-glaptop> <20120606184351.GA20380@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Cc: netdev@vger.kernel.org, linux-kernel@vger.kernel.org, virtualization@lists.linux-foundation.org, Stephen Hemminger To: "Michael S. Tsirkin" Return-path: In-Reply-To: <20120606184351.GA20380@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 List-Id: netdev.vger.kernel.org On Wed, 2012-06-06 at 21:43 +0300, Michael S. Tsirkin wrote: > 1. We are trying to look at counters for purposes of tuning the device. > E.g. if ethtool reports packets and bytes, we'd like to calculate > average packet size by bytes/packets. > > If both counters are read atomically the metric becomes more exact. > Not a must but nice to have. > metrics are exact right now. As soon as you read a value, it might already have changed. Maybe you want to stop_machine() to make sure all the metrics you want are 'exact' ;) > 2. 32 bit systems have some overhead because of the seqlock. > virtio could instead simply keep tx counters in the queue structure, and > get the tx lock when they are read. > But then you need atomic64 stuff, have you an idea of the cost of such primitives on 32bit ? 3. use 32bit counters on 32bit arches, as many drivers still do ?