netdev - Re: [PATCH net v2] napi: fix race inside napi

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Date:   Thu, 23 Sep 2021 07:54:18 -0700
From:   Jakub Kicinski <kuba@...nel.org>
To:     Xuan Zhuo <xuanzhuo@...ux.alibaba.com>
Cc:     netdev@...r.kernel.org, linyunsheng@...wei.com,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Neil Horman <nhorman@...hat.com>,
        Dust Li <dust.li@...ux.alibaba.com>,
        Wei Wang <weiwan@...gle.com>
Subject: Re: [PATCH net v2] napi: fix race inside napi_enable

On Thu, 23 Sep 2021 21:40:56 +0800 Xuan Zhuo wrote:
> On Thu, 23 Sep 2021 06:14:17 -0700, Jakub Kicinski <kuba@...nel.org> wrote:
> > > I think it should be an atomic operation. The original two-step clear itself is
> > > problematic. So from this perspective, it is not just a solution to this
> > > problem.  
> >
> > [resending, my MUA seems to have corrupted the CC list previously]
> >
> > Can you show what breaks by it being non-atomic?  
> 
> Isn't the problem this time caused by non-atoms?
> 
> Of course, in response to this problem, adjusting the order seems to be able to
> solve this problem. Compared to changing to atomic operations, we have to test
> other problems that may be caused by modifying this order.
> 
> Relatively speaking, the use of atoms is a relatively simple way of processing.

Whether atomics are simple or not is not the question.

What I'm saying is that having asymmetric enable and disable paths
is fragile.

> > Because, again, the disable part is not atomic. Either it's needed on
> > both sides or it's not needed on either.  
> 
> For the disable part, I think it’s okay not to use atoms. Have you considered
> any special scenarios?

The point is both sides should do the same thing.