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SubjectRe: [PATCH RFC] stop_machine: make stop_machine safe and efficient to call early
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);

_



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