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    SubjectRe: [PATCH RFC] pm_qos_requirement might sleep
    On Fri, Aug 15, 2008 at 12:51:11AM +0200, John Kacur wrote:
    > 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.

    It looks ok to me, do I need to add a compiler declaration to the
    structure to make sure the target_value is word aligned?

    thanks,

    --mgross


    >
    > Thanks
    >
    > John Kacur

    > 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);
    >



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