Subject: Re: [RFC] virtio_net: add local_bh_disable() around u64_stats_update_begin

From: Stephen Hemminger <[email-masked]>
Date: Tue, 16 Oct 2018 11:44:14 -0700

On Tue, 16 Oct 2018 20:42:07 +0200
Sebastian Andrzej Siewior <[email-masked]> wrote:

> On 2018-10-16 11:01:14 [-0700], Stephen Hemminger wrote:
> > On Tue, 16 Oct 2018 18:55:45 +0200
> > Sebastian Andrzej Siewior <[email-masked]> wrote:
> >   
> > > Also, ptr->var++ is not an atomic operation even on 64bit CPUs. Which
> > > means if try_fill_recv() runs on CPU0 (via virtnet_receive()) then the
> > > worker might run on CPU1.  
> > 
> > On modern CPU's increment of native types is atomic but not locked.
> > u64_stats_update_begin is a no-op on UP and also if BIT_PER_LONG != 32  
> 
> On ARM64 you have load, inc, store. So if two CPUs increment the counter
> simultaneously we might lose one increment. That is why I asked if we
> care or not.
> 
> Sebastian

The point is that kicks is just a counter, not important as part of the
device operation. The point of the u64_stats is to avoid problems with
high/low 32 bit wrap on increment. So this is ok on ARM64.