Re: [PATCH RFC] pm_qos_requirement might sleep

Peter Zijlstra
2008-08-25T16:35:42Z

On Mon, 2008-08-25 at 09:34 -0700, mark gross wrote:
> 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.
>
> I've been thinking about this patch and I worry that the readability
> from making the use of this lock asymmetric WRT reads and writes to the
> storage address is bothersome.
>
> I would rather make the variable an atomic. What do you think about
> that?

It would make the write side more expensive, as we already have the two
atomic operations for the lock and unlock, this would add a third.

Then again, I doubt that this is really a fast path.

OTOH, a simple comment could clarify the situation for the reader.

Up to you I guess ;-)