netdev - Re: [PATCH net-next 1/4] net: fix possible race in skb_attempt_defer

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Date:   Mon, 16 May 2022 11:15:54 -0700
From:   Jakub Kicinski <kuba@...nel.org>
To:     Eric Dumazet <eric.dumazet@...il.com>
Cc:     "David S . Miller" <davem@...emloft.net>,
        Paolo Abeni <pabeni@...hat.com>,
        netdev <netdev@...r.kernel.org>,
        Eric Dumazet <edumazet@...gle.com>
Subject: Re: [PATCH net-next 1/4] net: fix possible race in
 skb_attempt_defer_free()

On Sun, 15 May 2022 21:24:53 -0700 Eric Dumazet wrote:
> A cpu can observe sd->defer_count reaching 128,
> and call smp_call_function_single_async()
> 
> Problem is that the remote CPU can clear sd->defer_count
> before the IPI is run/acknowledged.
> 
> Other cpus can queue more packets and also decide
> to call smp_call_function_single_async() while the pending
> IPI was not yet delivered.
> 
> This is a common issue with smp_call_function_single_async().
> Callers must ensure correct synchronization and serialization.
> 
> I triggered this issue while experimenting smaller threshold.
> Performing the call to smp_call_function_single_async()
> under sd->defer_lock protection did not solve the problem.
> 
> Commit 5a18ceca6350 ("smp: Allow smp_call_function_single_async()
> to insert locked csd") replaced an informative WARN_ON_ONCE()
> with a return of -EBUSY, which is often ignored.
> Test of CSD_FLAG_LOCK presence is racy anyway.

If I'm reading this right this is useful for backports but in net-next
it really is a noop? The -EBUSY would be perfectly safe to ignore?
Just checking.