Messages in this thread | | | Subject | Re: [PATCH 00/11] Introduce Simple atomic and non-atomic counters | From | Shuah Khan <> | Date | Mon, 28 Sep 2020 17:01:55 -0600 |
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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
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