Date: Wed, 6 Jun 2012 22:58:15 +0300
From: "Michael S. Tsirkin" <mst@redhat.com>
To: Eric Dumazet <eric.dumazet@gmail.com>
Cc: Stephen Hemminger <shemminger@vyatta.com>,
        Jason Wang <jasowang@redhat.com>, netdev@vger.kernel.org,
        rusty@rustcorp.com.au, linux-kernel@vger.kernel.org,
        virtualization@lists.linux-foundation.org
Subject: Re: [PATCH] virtio-net: fix a race on 32bit arches
Message-ID: <20120606195814.GA20677@redhat.com>
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 <20120606111357.GA15070@redhat.com>
 <1338988210.2760.4485.camel@edumazet-glaptop>
 <20120606144941.GA17092@redhat.com>
 <20120606081432.6b602065@nehalam.linuxnetplumber.net>
 <20120606185107.GA20503@redhat.com>
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On Wed, Jun 06, 2012 at 09:54:01PM +0200, Eric Dumazet wrote:
> On Wed, 2012-06-06 at 21:51 +0300, Michael S. Tsirkin wrote:
> 
> > BTW for cards that do implement the counters in software,
> > under xmit lock, is anything wrong with simply taking the xmit lock
> > when we get the stats instead of the per-cpu trick + seqlock?
> > 
> 
> I still dont understand why you would do that.
> 
> Most modern machines are 64bits, so there is no seqlock overhead,
> nothing at all.
> 
> If you focus on 32bit hardware, just stick on 32bit counters ?

These wrap around.

> Note that most u64_stats_sync users are virtual drivers, without xmit
> lock (LLTX drivers)
> 
> 

Absolutely, I am talking about virtio here.  I'm not kicking
u64_stats_sync idea I am just saying that simple locking
would work for virtio and might be better as it
gives us a way to get counters atomically.


-- 
MST
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