Re: [PATCH RFC] pm_qos_requirement might sleep

["John Kacur" <jkacur@gmail.com>]

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On Thu, Aug 14, 2008 at 7:48 PM, Peter Zijlstra <peterz@infradead.org> wrote:
> On Thu, 2008-08-14 at 08:52 -0700, mark gross wrote:
>
>> Keeping a lock around the different "target_value"s may not be so
>> important.  Its just a 32bit scaler value, and perhaps we can make it an
>> atomic type?  That way we loose the raw_spinlock.
>
> My suggestion was to keep the locking for the write side - so as to
> avoid stuff stomping on one another, but drop the read side as:
>
>  spin_lock
>  foo = var;
>  spin_unlock
>  return foo;
>
> is kinda useless, it doesn't actually serialize against the usage of
> foo, that is, once it gets used, var might already have acquired a new
> value.
>
> The only thing it would protect is reading var, but since that is a
> machine sized read, its atomic anyway (assuming its naturally aligned).
>
> So no need for atomic_t (its read-side is just a read too), just drop
> the whole lock usage from pq_qos_requirement().
>

Thanks Peter.

Mark, is the following patch ok with you? This should be applied to
mainline, and then after that no special patches are necessary for
real-time.

Thanks

John Kacur

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Subject: Remove unnecessary lock in pm_qos_requirement

Signed-off-by: John Kacur <jkacur at gmail dot com>

Index: linux-2.6/kernel/pm_qos_params.c
===================================================================
--- linux-2.6.orig/kernel/pm_qos_params.c
+++ linux-2.6/kernel/pm_qos_params.c
@@ -193,14 +193,7 @@ static int find_pm_qos_object_by_minor(i
  */
 int pm_qos_requirement(int pm_qos_class)
 {
-	int ret_val;
-	unsigned long flags;
-
-	spin_lock_irqsave(&pm_qos_lock, flags);
-	ret_val = pm_qos_array[pm_qos_class]->target_value;
-	spin_unlock_irqrestore(&pm_qos_lock, flags);
-
-	return ret_val;
+	return pm_qos_array[pm_qos_class]->target_value;
 }
 EXPORT_SYMBOL_GPL(pm_qos_requirement);