linux-kernel - Re: [PATCH] smp,cpumask: Don't call functions on offline CPUs

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Date:   Wed, 22 May 2019 14:23:47 -0400
From:   Rik van Riel <riel@...riel.com>
To:     Peter Zijlstra <peterz@...radead.org>,
        Andrew Murray <andrew.murray@....com>
Cc:     Thomas Gleixner <tglx@...utronix.de>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] smp,cpumask: Don't call functions on offline CPUs

On Wed, 2019-05-22 at 16:49 +0200, Peter Zijlstra wrote:
> On Wed, May 22, 2019 at 03:37:11PM +0100, Andrew Murray wrote:
> > > Is perhaps the problem that on_each_cpu_cond() uses
> > > cpu_onlne_mask
> > > without protection?
> > 
> > Does this prevent racing with a CPU going offline? I guess this
> > prevents
> > the warning at the expense of a lock - but is only beneficial in
> > the
> > unlikely path. (In the likely path this prevents new CPUs going
> > offline
> > but we don't care because we don't WARN if they aren't they when we
> > attempt to call functions).
> > 
> > At least this is my limited understanding.
> 
> Hmm.. I don't think it could matter, we only use the mask when
> preempt_disable(), which would already block offline, due to it using
> stop_machine().
> 
> So the patch is a no-op.
> 
> What's the WARN you see? TLB invalidation should pass mm_cpumask(),
> which similarly should not contain offline CPUs I'm thinking.

Does the TLB invalidation code have anything in it
to prevent from racing with the CPU offline code?

In other words, could we end up with the TLB
invalidation code building its bitmask, getting
interrupted (eg. hypervisor preemption, NMI),
and not sending out the IPI to that bitmask of
CPUs until after one of the CPUs in the bitmap
has gotten offlined?

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