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

From: "Michael S. Tsirkin" <[email-masked]>
Date: Wed, 6 Jun 2012 19:17:15 +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.

Yes, of course.

> 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.
> 
> 

I understand, and not arguing about that.

But why do you say at most 1 packet?

Consider get_stats doing:
               u64_stats_update_begin(&stats->syncp);
               stats->tx_bytes += skb->len;

on 64 bit at this point
tx_packets might get incremented any number of times, no?

                stats->tx_packets++;
                u64_stats_update_end(&stats->syncp);

now tx_bytes and tx_packets are out of sync by more than 1.