Re: [PATCH] virtio-net: fix a race on 32bit arches

[Eric Dumazet]

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 ?



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