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

From: Shuah Khan <skhan@linuxfoundation.org>
To: Joel Fernandes <joel@joelfernandes.org>,
	Kees Cook <keescook@chromium.org>
Cc: corbet@lwn.net, gregkh@linuxfoundation.org, shuah@kernel.org,
	rafael@kernel.org, johannes@sipsolutions.net, lenb@kernel.org,
	james.morse@arm.com, tony.luck@intel.com, bp@alien8.de,
	arve@android.com, tkjos@android.com, maco@android.com,
	christian@brauner.io, hridya@google.com, surenb@google.com,
	minyard@acm.org, arnd@arndb.de, mchehab@kernel.org,
	rric@kernel.org, linux-doc@vger.kernel.org,
	linux-kernel@vger.kernel.org, linux-kselftest@vger.kernel.org,
	linux-acpi@vger.kernel.org, devel@driverdev.osuosl.org,
	openipmi-developer@lists.sourceforge.net,
	linux-edac@vger.kernel.org,
	Shuah Khan <skhan@linuxfoundation.org>
Subject: Re: [PATCH 00/11] Introduce Simple atomic and non-atomic counters
Date: Mon, 28 Sep 2020 17:01:55 -0600	[thread overview]
Message-ID: <9c237ead-5b68-2e3f-2af6-a08c03b24fde@linuxfoundation.org> (raw)
In-Reply-To: <20200928211709.GA2641213@google.com>

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