linux-kernel - Re: [RFC][PATCH 12/13] stop

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Message-ID: <20150623100932.GB3644@twins.programming.kicks-ass.net>
Date:	Tue, 23 Jun 2015 12:09:32 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	paulmck@...ux.vnet.ibm.com, tj@...nel.org, mingo@...hat.com,
	linux-kernel@...r.kernel.org, der.herr@...r.at, dave@...olabs.net,
	riel@...hat.com, viro@...IV.linux.org.uk,
	torvalds@...ux-foundation.org
Subject: Re: [RFC][PATCH 12/13] stop_machine: Remove lglock

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.
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