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Date: Mon, 23 Sep 2019 17:48:52 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Michal Hocko <mhocko@kernel.org>
Subject: Re: [PATCH v6] numa: make node_to_cpumask_map() NUMA_NO_NODE aware
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On Mon, Sep 23, 2019 at 05:28:56PM +0200, Michal Hocko wrote:
> On Mon 23-09-19 17:15:19, Peter Zijlstra wrote:

> > > diff --git a/arch/x86/mm/numa.c b/arch/x86/mm/numa.c
> > > index 4123100e..9859acb 100644
> > > --- a/arch/x86/mm/numa.c
> > > +++ b/arch/x86/mm/numa.c
> > > @@ -861,6 +861,9 @@ void numa_remove_cpu(int cpu)
> > >   */
> > >  const struct cpumask *cpumask_of_node(int node)
> > >  {
> > > +	if (node == NUMA_NO_NODE)
> > > +		return cpu_online_mask;
> > 
> > This mandates the caller holds cpus_read_lock() or something, I'm pretty
> > sure that if I put:
> > 
> > 	lockdep_assert_cpus_held();
> 
> Is this documented somewhere?

No idea... common sense :-)

> Also how does that differ from a normal
> case when a proper node is used? The cpumask will always be dynamic in
> the cpu hotplug presence, right?

As per normal yes, and I'm fairly sure there's a ton of bugs. Any
'online' state is subject to change except when you're holding
sufficient locks to stop it.

Disabling preemption also stabilizes it, because cpu unplug relies on
stop-machine.

> > here, it comes apart real quick. Without holding the cpu hotplug lock,
> > the online mask is gibberish.
> 
> Can the returned cpu mask go away?

No, the cpu_online_mask itself has static storage, the contents OTOH can
change at will. Very little practical difference :-)