linux-kernel - Re: [PATCH 2/3] work_on_cpu: Use our own workqueue.

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Message-Id: <200901271745.19574.rusty@rustcorp.com.au>
Date:	Tue, 27 Jan 2009 17:45:18 +1030
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Oleg Nesterov <oleg@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	a.p.zijlstra@...llo.nl, travis@....com, mingo@...hat.com,
	davej@...hat.com, cpufreq@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/3] work_on_cpu: Use our own workqueue.

On Tuesday 27 January 2009 08:47:29 Ingo Molnar wrote:
> 
> * Oleg Nesterov <oleg@...hat.com> wrote:
> 
> > But "[PATCH 1/3] work_on_cpu: dont try to get_online_cpus() in 
> > work_on_cpu." removes get_online_cpus/put_online_cpus, this means the 
> > work can run on the wrong CPU anyway. Or work_on_cpu() can hang forever 
> > if CPU has already gone away before queue_work_on().
> > 
> > Confused.
> 
> The idea was to require work_on_cpu() users to be CPU hotplug-safe. But 
> ... Rusty pointed it out in the past that this might be fragile, and we 
> could put back the get_online_cpus()/put_online_cpus() calls.

Old code used to do:

	tmp = current->cpus_allowed;
	set_cpus_allowed(current, cpumask_of_cpu(cpu));
	function(arg);
	set_cpus_allowed(current, tmp);

We replaced it with:

	work_on_cpu(cpu, function, arg);

I thought I'd be clever and reliably check that the cpu they asked for
was online inside work_on_cpu.  Leading to locking problems.  But if they
didn't previously ensure cpu hotplug didn't happen, they were buggy already,
so I took out the check and hence the hotplug lock.

So we're no *worse* than we were before, but yes, an audit would probably
lead to fixes.

Hope that clarifies?
Rusty.
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