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

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Date:	Fri, 30 Sep 2011 17:21:51 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Andrew Morton <akpm00@...il.com>
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, 2011-09-30 at 14:06 -0700, Andrew Morton wrote:
> On Fri, 30 Sep 2011 10:00:06 -0700
> Jeremy Fitzhardinge <jeremy@...p.org> wrote:
> 
> > On 09/30/2011 09:34 AM, Steven Rostedt wrote:
> > > Doesn't interrupts need to be disabled here too? As stop machine
> > > functions also guarantee that they will not be interrupted by
> > > interrupts. -- Steve
> > 
> > Good point.
> > 
> >     J
> > 
> > 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.
> 
> Being a trusting soul, I shall assume that you presently have or soon
> will have some code which requires this change?

Yeah, it has to do with these patches:

https://lkml.org/lkml/2011/9/29/509

> 
> >
> > ...
> >
> > --- a/kernel/stop_machine.c
> > +++ b/kernel/stop_machine.c
> > @@ -485,6 +485,17 @@ 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) {
> > +		unsigned long flags;
> > +		int ret;
> > +
> > +		local_save_flags(flags);
> > +		ret = (*fn)(data);
> > +		local_irq_restore(flags);
> > +
> > +		return ret;
> > +	}
> 
> It is quite unobvious to readers why this code exists.  Therefore, we
> should tell them.
> 
> Also, bug.  local_save_flags() is used to read the irq flags - it does
> not actually disable interrupts.  We want local_irq_save() here.  None
> of these interfaces are documented and their naming system is crappy. 
> Cause, effect.
> 
> 
> --- a/kernel/stop_machine.c~stop_machine-make-stop_machine-safe-and-efficient-to-call-early-fix
> +++ a/kernel/stop_machine.c
> @@ -486,10 +486,14 @@ int __stop_machine(int (*fn)(void *), vo
>  					    .active_cpus = cpus };
>  
>  	if (smdata.num_threads == 1) {
> +		/*
> +		 * Handle the case where stop_machine() is called early in boot
> +		 * before SMP startup.
> +		 */
>  		unsigned long flags;
>  		int ret;
>  
> -		local_save_flags(flags);
> +		local_irq_save(flags);

Nice catch. Yeah, the naming convention is not too nice. They probably
should have been named: local_irq_disable_save() and
local_irq_enable_restore(). But that's too much to type ;)

-- Steve


>  		ret = (*fn)(data);
>  		local_irq_restore(flags);
>  
> _
> 


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