netdev - Re: [RFC] virtio_net: add local_bh_disable() around u64_stats_update

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Date:   Tue, 16 Oct 2018 11:44:14 -0700
From:   Stephen Hemminger <stephen@...workplumber.org>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:     netdev@...r.kernel.org, virtualization@...ts.linux-foundation.org,
        tglx@...utronix.de,
        Toshiaki Makita <makita.toshiaki@....ntt.co.jp>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Jason Wang <jasowang@...hat.com>,
        "David S. Miller" <davem@...emloft.net>
Subject: Re: [RFC] virtio_net: add local_bh_disable() around
 u64_stats_update_begin

On Tue, 16 Oct 2018 20:42:07 +0200
Sebastian Andrzej Siewior <bigeasy@...utronix.de> wrote:

> On 2018-10-16 11:01:14 [-0700], Stephen Hemminger wrote:
> > On Tue, 16 Oct 2018 18:55:45 +0200
> > Sebastian Andrzej Siewior <bigeasy@...utronix.de> 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.