linux-kernel - Re: [RFC PATCH 01/11] counters: Introduce counter and counter

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Date:   Wed, 23 Sep 2020 15:19:08 -0600
From:   Shuah Khan <skhan@...uxfoundation.org>
To:     Kees Cook <keescook@...omium.org>
Cc:     corbet@....net, gregkh@...uxfoundation.org,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
        Shuah Khan <skhan@...uxfoundation.org>
Subject: Re: [RFC PATCH 01/11] counters: Introduce counter and counter_atomic

On 9/23/20 2:58 PM, Kees Cook wrote:
> On Wed, Sep 23, 2020 at 02:48:22PM -0600, Shuah Khan wrote:
>> On 9/23/20 1:04 PM, Kees Cook wrote:
>>> On Tue, Sep 22, 2020 at 07:43:30PM -0600, Shuah Khan wrote:
>>> I would really like these APIs to be _impossible_ to use for object
>>> lifetime management. To that end, I would like to have all of the
>>> *_return() functions removed. It should be strictly init, inc, dec,
>>> read.
>>>
>>
>> Yes. I am with you on making this API as small as possible so it won't
>> be used for lifetime mgmt. That means no support for:
>>
>> *_test, add_negative etc.
>>
>> I started out with just init, inc, dec, read. As I started looking
>> for candidates that can be converted to counters, I found inc_return()
>> usages. I think we need inc_return() for sure. I haven't come across
>> atomic_dec_return() yet.
> 
> What are the inc_return() cases? If they're not "safe" to use inc() and
> then read(), then those likely need a closer look at what they're doing.
> 

3 in this series I sent. I would say I barely scratched the surface
when it comes to finding candidates for converting.

drivers/android/binder.c
drivers/acpi/acpi_extlog.c
drivers/acpi/apei/ghes.c

These uses look reasonable to me. Having this inc_return() will save
making _inc() followed by _read()

>>>> +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.
>>>
>>> Why even force the distinction? I think all the counters should be
>>> atomic and then there is no chance they will get accidentally used in
>>> places where someone *thinks* it's safe to use a non-atomic. So,
>>> "_atomic" can be removed from the name and the non-atomic implementation
>>> can get removed. Anyone already using non-atomic counters is just using
>>> "int" and "long" anyway. Let's please only create APIs that are always
>>> safe to use, and provide some benefit over a native time.
>>>
>>
>> I am with Greg on this. I think we will find several atomic_t usages
>> that don't need atomicity.
> 
> If you want to distinguish from atomic and create a wrapping "int", how
> about making "counter" be the atomic and name the other "counter_unsafe"
> (or "counter_best_effort", "counter_simple", ...) etc?
> 

I will change counter to counter_simple and add a warning that this
should only be used when atomic isn't needed. I can outline some
tips for choosing the right one.

thanks,
-- Shuah