Re: [PATCH] cpu/hotplug: Cache number of online CPUs

[Mathieu Desnoyers]

----- On Jul 4, 2019, at 6:33 PM, Thomas Gleixner tglx@xxxxxxxxxxxxx wrote:

> On Thu, 4 Jul 2019, Mathieu Desnoyers wrote:
>> ----- On Jul 4, 2019, at 5:10 PM, Thomas Gleixner tglx@xxxxxxxxxxxxx wrote:
>> >
>> > num_online_cpus() is racy today vs. CPU hotplug operations as
>> > long as you don't hold the hotplug lock.
>> 
>> Fair point, AFAIU none of the loads performed within num_online_cpus()
>> seem to rely on atomic nor volatile accesses. So not using a volatile
>> access to load the cached value should not introduce any regression.
>> 
>> I'm concerned that some code may rely on re-fetching of the cached
>> value between iterations of a loop. The lack of READ_ONCE() would
>> let the compiler keep a lifted load within a register and never
>> re-fetch, unless there is a cpu_relax() or a barrier() within the
>> loop.
> 
> If someone really wants to write code which can handle concurrent CPU
> hotplug operations and rely on that information, then it's probably better
> to write out:
> 
>     ncpus = READ_ONCE(__num_online_cpus);
> 
> explicitely along with a big fat comment.
> 
> I can't figure out why one wants to do that and how it is supposed to work,
> but my brain is in shutdown mode already :)
> 
> I'd rather write a proper kernel doc comment for num_online_cpus() which
> explains what the constraints are instead of pretending that the READ_ONCE
> in the inline has any meaning.

The other aspect I am concerned about is freedom given to the compiler
to perform the store to __num_online_cpus non-atomically, or the load
non-atomically due to memory pressure. Is that something we should be
concerned about ?

I thought we had WRITE_ONCE and READ_ONCE to take care of that kind of
situation.

The semantic I am looking for here is C11's relaxed atomics.

Thanks,

Mathieu


> 
> Thanks,
> 
> 	tglx

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com