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

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Message-ID: <611100399.5550.1562283294601.JavaMail.zimbra@efficios.com>
Date:   Thu, 4 Jul 2019 19:34:54 -0400 (EDT)
From:   Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     linux-kernel <linux-kernel@...r.kernel.org>, x86 <x86@...nel.org>,
        Nadav Amit <namit@...are.com>,
        "Paul E. McKenney" <paulmck@...ux.ibm.com>
Subject: Re: [PATCH] cpu/hotplug: Cache number of online CPUs

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

> On Thu, 4 Jul 2019, Mathieu Desnoyers wrote:
>> ----- On Jul 4, 2019, at 5:10 PM, Thomas Gleixner tglx@...utronix.de 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