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

[Andrew Morton <akpm@zip.com.au>]

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)

 
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