netdev - Re: [PATCH net-next] tcp: use 2-arg optimal variant of kfree

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Date:   Fri, 2 Dec 2022 19:28:31 -0500
From:   Joel Fernandes <joel@...lfernandes.org>
To:     paulmck@...nel.org
Cc:     Eric Dumazet <edumazet@...gle.com>,
        "David S . Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>, netdev@...r.kernel.org,
        eric.dumazet@...il.com, Dmitry Safonov <dima@...sta.com>,
        rcu@...r.kernel.org
Subject: Re: [PATCH net-next] tcp: use 2-arg optimal variant of kfree_rcu()

+rcu for archives 

> On Dec 2, 2022, at 7:16 PM, Joel Fernandes <joel@...lfernandes.org> wrote:
> 
> On Sat, Dec 3, 2022 at 12:12 AM Joel Fernandes <joel@...lfernandes.org> wrote:
>> 
>>> On Sat, Dec 3, 2022 at 12:03 AM Paul E. McKenney <paulmck@...nel.org> wrote:
>>> 
>>> On Fri, Dec 02, 2022 at 11:49:59PM +0000, Joel Fernandes wrote:
>>>> On Fri, Dec 02, 2022 at 05:28:47AM +0000, Eric Dumazet wrote:
>>>>> kfree_rcu(1-arg) should be avoided as much as possible,
>>>>> since this is only possible from sleepable contexts,
>>>>> and incurr extra rcu barriers.
>>>>> 
>>>>> I wish the 1-arg variant of kfree_rcu() would
>>>>> get a distinct name, like kfree_rcu_slow()
>>>>> to avoid it being abused.
>>>> 
>>>> Hi Eric,
>>>> Nice to see your patch.
>>>> 
>>>> Paul, all, regarding Eric's concern, would the following work to warn of
>>>> users? Credit to Paul/others for discussing the idea on another thread. One
>>>> thing to note here is, this debugging will only be in effect on preemptible
>>>> kernels, but should still help catch issues hopefully.
>>> 
>>> Mightn't there be some places where someone needs to invoke
>>> single-argument kfree_rcu() in a preemptible context, for example,
>>> due to the RCU-protected structure being very small and very numerous?
>> 
>> This could be possible but I am not able to find examples of such
>> cases, at the moment. Another approach could be to introduce a
>> dedicated API for such cases, where the warning will not fire. And
>> keep the warning otherwise.
>> 
>> Example: kfree_rcu_headless()
>> With a big comment saying, use only if you are calling from a
>> preemptible context and cannot absolutely embed an rcu_head. :-)
>> 
>> Thoughts?
>> 
> 
> Just to clarify, where I was getting at was to combine both ideas:
> 1. new API with suppression of the new warning mentioned above.
> 2. old API but add new warning mentioned above.
> 
> Cheers,
> 
> - Joel