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

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Message-ID: <20181016184206.coukhtgmlr32hyl7@linutronix.de>
Date:   Tue, 16 Oct 2018 20:42:07 +0200
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     Stephen Hemminger <stephen@...workplumber.org>
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 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