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

From: "Michael S. Tsirkin" <[email-masked]>
Date: Wed, 6 Jun 2012 19:57:55 +0300

On Wed, Jun 06, 2012 at 05:19:04PM +0200, Eric Dumazet wrote:
> 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.
> 

So for virtio since all counters get incremented from bh we can
ensure they are read atomically, simply but reading them
from the correct CPU with bh disabled.
And then we don't need u64_stats_sync at all.

-- 
MST