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

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Date:	Tue, 23 Jun 2015 00:21:52 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Peter Zijlstra <peterz@...radead.org>
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 06/22, Peter Zijlstra wrote:
>
> By having stop_two_cpus() acquire two cpu_stopper::locks we gain full
> order against the global stop_machine which takes each of these locks
> in order.

Yes, but stop_machine() locks/unlocs cpu_stopper->lock sequentially, it
never holds more than 1 ->lock, so

> +static void cpu_stop_queue_work(unsigned int cpu, struct cpu_stop_work *work)
> +{
> +	struct cpu_stopper *stopper = &per_cpu(cpu_stopper, cpu);
> +	unsigned long flags;
> +
> +	spin_lock_irqsave(&stopper->lock, flags);
> +	__cpu_stop_queue_work(cpu, work);
>  	spin_unlock_irqrestore(&stopper->lock, flags);
>  }

...

>  int stop_two_cpus(unsigned int cpu1, unsigned int cpu2, cpu_stop_fn_t fn, void *arg)
>  {
> -	struct cpu_stop_done done;
> +	struct cpu_stopper *stopper1, *stopper2;
>  	struct cpu_stop_work work1, work2;
>  	struct multi_stop_data msdata;
> +	struct cpu_stop_done done;
> +	unsigned long flags;
> +
> +	if (cpu2 < cpu1)
> +		swap(cpu1, cpu2);

...

> +	stopper1 = per_cpu_ptr(&cpu_stopper, cpu1);
> +	stopper2 = per_cpu_ptr(&cpu_stopper, cpu2);
> +
> +	spin_lock_irqsave(&stopper1->lock, flags);
> +	spin_lock(&stopper2->lock);
> +
> +	__cpu_stop_queue_work(cpu1, &work1);
> +	__cpu_stop_queue_work(cpu2, &work2);

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

	- 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?

Oleg.

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