linux-kernel - Re: [PATCH 00/11] Introduce Simple atomic and non-atomic counters

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Date:   Mon, 28 Sep 2020 17:01:55 -0600
From:   Shuah Khan <skhan@...uxfoundation.org>
To:     Joel Fernandes <joel@...lfernandes.org>,
        Kees Cook <keescook@...omium.org>
Cc:     corbet@....net, gregkh@...uxfoundation.org, shuah@...nel.org,
        rafael@...nel.org, johannes@...solutions.net, lenb@...nel.org,
        james.morse@....com, tony.luck@...el.com, bp@...en8.de,
        arve@...roid.com, tkjos@...roid.com, maco@...roid.com,
        christian@...uner.io, hridya@...gle.com, surenb@...gle.com,
        minyard@....org, arnd@...db.de, mchehab@...nel.org,
        rric@...nel.org, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org,
        linux-acpi@...r.kernel.org, devel@...verdev.osuosl.org,
        openipmi-developer@...ts.sourceforge.net,
        linux-edac@...r.kernel.org, Shuah Khan <skhan@...uxfoundation.org>
Subject: Re: [PATCH 00/11] Introduce Simple atomic and non-atomic counters

On 9/28/20 3:17 PM, Joel Fernandes wrote:
> On Mon, Sep 28, 2020 at 01:34:31PM -0700, Kees Cook wrote:
>> On Sun, Sep 27, 2020 at 07:35:26PM -0400, Joel Fernandes wrote:
>>> On Fri, Sep 25, 2020 at 05:47:14PM -0600, Shuah Khan wrote:
>>>> This patch series is a result of discussion at the refcount_t BOF
>>>> the Linux Plumbers Conference. In this discussion, we identified
>>>> a need for looking closely and investigating atomic_t usages in
>>>> the kernel when it is used strictly as a counter without it
>>>> controlling object lifetimes and state changes.
>>>>
>>>> There are a number of atomic_t usages in the kernel where atomic_t api
>>>> is used strictly for counting and not for managing object lifetime. In
>>>> some cases, atomic_t might not even be needed.
>>>>      
>>>> The purpose of these counters is twofold: 1. clearly differentiate
>>>> atomic_t counters from atomic_t usages that guard object lifetimes,
>>>> hence prone to overflow and underflow errors. It allows tools that scan
>>>> for underflow and overflow on atomic_t usages to detect overflow and
>>>> underflows to scan just the cases that are prone to errors. 2. provides
>>>> non-atomic counters for cases where atomic isn't necessary.
>>>
>>> Nice series :)
>>>

Thanks.

>>> It appears there is no user of counter_simple in this series other than the
>>> selftest. Would you be planning to add any conversions in the series itself,
>>> for illustration of use? Sorry if I missed a usage.
>>>
>>> Also how do we guard against atomicity of counter_simple RMW operations? Is
>>> the implication that it should be guarded using other synchronization to
>>> prevent lost-update problem?
>>>
>>> Some more comments:
>>>
>>> 1.  atomic RMW operations that have a return value are fully ordered. Would
>>>      you be adding support to counter_simple for such ordering as well, for
>>>      consistency?
>>
>> No -- there is no atomicity guarantee for counter_simple. I would prefer
>> counter_simple not exist at all, specifically for this reason.
> 
> Yeah I am ok with it not existing, especially also as there are no examples
> of its conversion/usage in the series.
> 

No. counter_simple is just for counting when there is no need for
atomicity with the premise that there might be some use-cases. You
are right that this patch series doesn't use these. My hunch is though
that atomic_t is overused and it isn't needed in all cases.

I will do some research to look for any places that can use
counter_simple before I spin v2. If I don't find any, I can drop them.

thanks,
-- Shuah