linux-kernel - Re: [PATCH RFC] stop_machine: make stop_machine safe and efficient to call early

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Message-Id: <20110930142304.c455799e.akpm00@gmail.com>
Date:	Fri, 30 Sep 2011 14:23:04 -0700
From:	Andrew Morton <akpm00@...il.com>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Jeremy Fitzhardinge <jeremy@...p.org>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	"H. Peter Anvin" <hpa@...or.com>, Tejun Heo <htejun@...il.com>,
	Ingo Molnar <mingo@...hat.com>
Subject: Re: [PATCH RFC] stop_machine: make stop_machine safe and efficient
 to call early

On Fri, 30 Sep 2011 12:34:54 -0400
Steven Rostedt <rostedt@...dmis.org> wrote:

> On Fri, 2011-09-30 at 09:28 -0700, Jeremy Fitzhardinge wrote:
> > Make stop_machine() safe to call early in boot, before SMP has been
> > set up, by simply calling the callback function directly if there's
> > only one CPU online.
> > 
> > Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@...rix.com>
> > Cc: Tejun Heo <tj@...nel.org>
> > Cc: Rusty Russell <rusty@...tcorp.com.au>
> > Cc: Peter Zijlstra <a.p.zijlstra@...llo.nl>
> > Cc: Andrew Morton <akpm@...ux-foundation.org>
> > Cc: H. Peter Anvin <hpa@...ux.intel.com>
> > Cc: Ingo Molnar <mingo@...e.hu>
> > Cc: Steven Rostedt <rostedt@...dmis.org>
> > 
> > diff --git a/kernel/stop_machine.c b/kernel/stop_machine.c
> > index ba5070c..2df15ca 100644
> > --- a/kernel/stop_machine.c
> > +++ b/kernel/stop_machine.c
> > @@ -485,6 +485,9 @@ int __stop_machine(int (*fn)(void *), void *data, const struct cpumask *cpus)
> >  					    .num_threads = num_online_cpus(),
> >  					    .active_cpus = cpus };
> >  
> > +	if (smdata.num_threads == 1)
> > +		return (*fn)(data);
> 
> Doesn't interrupts need to be disabled here too? As stop machine
> functions also guarantee that they will not be interrupted by
> interrupts.
> 

If we wish to truly emulate the stop_machine_cpu_stop() callback
environment then we should run hard_irq_disable() as well?

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