Messages in this thread | | | Date | Fri, 5 Jul 2019 00:33:23 +0200 (CEST) | From | Thomas Gleixner <> | Subject | Re: [PATCH] cpu/hotplug: Cache number of online CPUs |
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On Thu, 4 Jul 2019, Mathieu Desnoyers wrote: > ----- On Jul 4, 2019, at 5:10 PM, Thomas Gleixner tglx@linutronix.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.
Thanks,
tglx
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