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Date: Thu, 25 Jun 2015 21:18:02 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Oleg Nesterov <oleg@redhat.com>
Cc: paulmck@linux.vnet.ibm.com, tj@kernel.org, mingo@redhat.com,
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        torvalds@linux-foundation.org
Subject: Re: [RFC][PATCH 12/13] stop_machine: Remove lglock
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On Tue, Jun 23, 2015 at 07:24:16PM +0200, Oleg Nesterov wrote:
> IOW. Suppose we add ->work_mutex into struct cpu_stopper. Btw,
> I think we should move all per-cpu variables there...
> 
> Now,
> 
> 	lock_stop_cpus_works(cpumask)
> 	{
> 		for_each_cpu(cpu, cpumask)
> 			mutex_lock(per_cpu(cpu_stopper_task, cpu).work_mutex);
> 	}
> 
> 	unlock_stop_cpus_works(cpumask)
> 	{
> 		for_each_cpu(cpu, cpumask)
> 			mutex_lock(...);
> 	}
> 
> which should be used instead of stop_cpus_mutex. After this change
> stop_two_cpus() can just use stop_cpus().

Right, lockdep annotating that will be 'interesting' though. And
stop_two_cpus() then has the problem of allocating a cpumask. Simpler to
let it keep 'abuse' the queueing spinlock in there.

> Off-topic. Can't we make __stop_machine() static? The only caller,
> _cpu_down() can safely call stop_machine(), get_online_cpus() is
> fine under cpu_hotplug_begin().

Can do I think.