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Date: Fri, 30 Sep 2011 14:06:25 -0700
From: Andrew Morton <akpm00@gmail.com>
To: Jeremy Fitzhardinge <jeremy@goop.org>
Cc: Steven Rostedt <rostedt@goodmis.org>,
        Rusty Russell <rusty@rustcorp.com.au>,
        Peter Zijlstra <a.p.zijlstra@chello.nl>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        "H. Peter Anvin" <hpa@zytor.com>, Tejun Heo <htejun@gmail.com>,
        Ingo Molnar <mingo@redhat.com>
Subject: Re: [PATCH RFC] stop_machine: make stop_machine safe and efficient
 to call early
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On Fri, 30 Sep 2011 10:00:06 -0700
Jeremy Fitzhardinge <jeremy@goop.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?

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