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

[Peter Zijlstra]

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.