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

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Date:   Fri, 19 Oct 2018 10:19:56 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     David Miller <davem@...emloft.net>, bigeasy@...utronix.de
Cc:     stephen@...workplumber.org, netdev@...r.kernel.org,
        virtualization@...ts.linux-foundation.org, tglx@...utronix.de,
        makita.toshiaki@....ntt.co.jp, mst@...hat.com
Subject: Re: [PATCH] virtio_net: add local_bh_disable() around
 u64_stats_update_begin


On 2018/10/19 上午7:23, David Miller wrote:
> From: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
> Date: Thu, 18 Oct 2018 10:43:13 +0200
>
>> on 32bit, lockdep notices that virtnet_open() and refill_work() invoke
>> try_fill_recv() from process context while virtnet_receive() invokes the
>> same function from BH context. The problem that the seqcounter within
>> u64_stats_update_begin() may deadlock if it is interrupted by BH and
>> then acquired again.
>>
>> Introduce u64_stats_update_begin_bh() which disables BH on 32bit
>> architectures. Since the BH might interrupt the worker, this new
>> function should not limited to SMP like the others which are expected
>> to be used in softirq.
>>
>> With this change we might lose increments but this is okay. The
>> important part that the two 32bit parts of the 64bit counter are not
>> corrupted.
>>
>> Fixes: 461f03dc99cf6 ("virtio_net: Add kick stats").
>> Suggested-by: Stephen Hemminger <stephen@...workplumber.org>
>> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
> Trying to get down to the bottom of this:
>
> 1) virtnet_receive() runs from softirq but only if NAPI is active and
>     enabled.  It is in this context that it invokes try_fill_recv().
>
> 2) refill_work() runs from process context, but disables NAPI (and
>     thus invocation of virtnet_receive()) before calling
>     try_fill_recv().
>
> 3) virtnet_open() invokes from process context as well, but before the
>     NAPI instances are enabled, it is same as case #2.
>
> 4) virtnet_restore_up() is the same situations as #3.
>
> Therefore I agree that this is a false positive, and simply lockdep
> cannot see the NAPI synchronization done by case #2.
>
> I think we shouldn't add unnecessary BH disabling here, and instead
> find some way to annotate this for lockdep's sake.
>
> Thank you.
>

+1

Thanks