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Message-ID: <20190705084910.GA6592@gmail.com>
Date: Fri, 5 Jul 2019 10:49:10 +0200
From: Ingo Molnar <mingo@...nel.org>
To: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc: Thomas Gleixner <tglx@...utronix.de>,
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
* Mathieu Desnoyers <mathieu.desnoyers@...icios.com> wrote:
> ----- 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.
What connection does "memory pressure" have to what the compiler does?
Did you confuse it with "register pressure"?
> Is that something we should be concerned about ?
Once I understand it :)
> I thought we had WRITE_ONCE and READ_ONCE to take care of that kind of
> situation.
Store and load tearing is one of the minor properties of READ_ONCE() and
WRITE_ONCE() - the main properties are the ordering guarantees.
Since __num_online_cpus is neither weirdly aligned nor is it written via
constants I don't see how load/store tearing could occur. Can you outline
such a scenario?
> The semantic I am looking for here is C11's relaxed atomics.
What does this mean?
Thanks,
Ingo
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