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Date: Tue, 23 Jun 2015 12:09:32 +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 12:21:52AM +0200, Oleg Nesterov wrote:

> Suppose that stop_two_cpus(cpu1 => 0, cpu2 => 1) races with stop_machine().
> 
> 	- stop_machine takes the lock on CPU 0, adds the work
> 	  and drops the lock
> 
> 	- cpu_stop_queue_work() queues both works

cpu_stop_queue_work() only ever queues _1_ work.

> 	- stop_machine takes the lock on CPU 1, etc
> 
> In this case both CPU 0 and 1 will run multi_cpu_stop() but they will
> use different multi_stop_data's, so they will wait for each other
> forever?

So what you're saying is:

	queue_stop_cpus_work()		stop_two_cpus()

	cpu_stop_queue_work(0,..);
					spin_lock(0);
					spin_lock(1);

					__cpu_stop_queue_work(0,..);
					__cpu_stop_queue_work(1,..);

					spin_unlock(1);
					spin_unlock(0);
	cpu_stop_queue_work(1,..);

Indeed, I don't know what I was thinking...

We can of course slap a percpu-rwsem in, but I wonder if there's
anything smarter we can do here.