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

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Date:	Wed, 06 Jun 2012 17:19:04 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	"Michael S. Tsirkin" <mst@...hat.com>
Cc:	Jason Wang <jasowang@...hat.com>, netdev@...r.kernel.org,
	rusty@...tcorp.com.au, linux-kernel@...r.kernel.org,
	virtualization@...ts.linux-foundation.org,
	Stephen Hemminger <shemminger@...tta.com>
Subject: Re: [PATCH] virtio-net: fix a race on 32bit arches

On Wed, 2012-06-06 at 17:49 +0300, Michael S. Tsirkin wrote:
> On Wed, Jun 06, 2012 at 03:10:10PM +0200, Eric Dumazet wrote:
> > On Wed, 2012-06-06 at 14:13 +0300, Michael S. Tsirkin wrote:
> > 
> > > We currently do all stats either on napi callback or from
> > > start_xmit callback.
> > > This makes them safe, yes?
> > 
> > Hmm, then _bh() variant is needed in virtnet_stats(), as explained in
> > include/linux/u64_stats_sync.h section 6)
> > 
> >  * 6) If counter might be written by an interrupt, readers should block interrupts.
> >  *    (On UP, there is no seqcount_t protection, a reader allowing interrupts could
> >  *     read partial values)
> > 
> > Yes, its tricky...
> 
> Sounds good, but I have a question: this realies on counters
> being atomic on 64 bit.
> Would not it be better to always use a seqlock even on 64 bit?
> This way counters would actually be correct and in sync.
> As it is if we want e.g. average packet size,
> we can not rely e.g. on it being bytes/packets.

When this stuff was discussed, we chose to have a nop on 64bits.

Your point has little to do with 64bit stats, it was already like that
with 'long int' counters.

Consider average driver doing :

dev->stats.rx_bytes += skb->len;
dev->stats.rx_packets++;

A concurrent reader can read an updated rx_bytes and a 'previous'
rx_packets one.

'fixing' this requires a lot of work and memory barriers (in all
drivers), for a very litle gain (at most one packet error)

u64_stats_sync was really meant to be 0-cost on 64bit arches.



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