Re: [PATCH] 2.5.3 remove global semaphore_lock spin lock.

From: Bob Miller (rem@osdl.org)
Date: Fri Feb 01 2002 - 15:52:34 EST

On Thu, Jan 31, 2002 at 03:55:02PM -0800, Andrew Morton wrote:
> Bob Miller wrote:
> >
> > Below is a patch for i386 that replaces the global spin lock semaphore_lock,
> > with the rwlock_t embedded in the wait_queue_head_t in the struct semaphore.
> >
>
> Looks sane. In practice, the speedup is unmeasurable, but...
>
> > ...
> > + unsigned long flags;
> > + wq_write_lock_irqsave(&sem->wait.lock, flags);
> > - spin_lock_irq(&semaphore_lock);
>
> I rather dislike spin_lock_irq(), because it's fragile (makes
> assumptions about the caller's state). But in this case,
> it's probably a reasonable micro-optimisation to not have to
> save the flags. Nobody should be calling down() with local
> interrupts disabled.
>
> > ...
> > +/*
> > + * Same as __wake_up but called with the wait_queue_head_t lock held
> > + * in at least read mode.
> > + */
> > +void __wake_up_locked(wait_queue_head_t *q, unsigned int mode, int nr)
> > +{
> > + if (q) {
>
> I don't think we need to test `q' here. It's a new function,
> and we don't need to support broken callers. So __wake_up_locked()
> can become a macro direct call to __wake_up_common().
>
> > + __wake_up_common(q, mode, nr, 0);
>
> This one breaks the camel's back :)
>
> Let's un-inline __wake_up_common and EXPORT_SYMBOL it.
>
> It'd be good if you could also verify that the code still
> works when the use-rwlocks-for-waitqueues option is turned
> on. (wait.h:USE_RW_WAIT_QUEUE_SPINLOCK)
>
>
Thanks for the feed back. I've incorporated your comments. Also, at your
suggestion I set wait.h:USE_RW_WAIT_QUEUE_SPINLOCK on a clean 2.5.3 system
to test. The problem is that it OOPs on startup. After I track that down
and test with my stuff I'll resubmit the patch.

Thanks for taking the time... 

-- 
Bob Miller					Email: rem@osdl.org
Open Software Development Lab			Phone: 503.626.2455 Ext. 17
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