linux-kernel - Re: [patch 09/10] SLUB: Do our own locking via slab_lock and slab

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Date:	Sun, 28 Oct 2007 20:03:12 -0700 (PDT)
From:	Christoph Lameter <clameter@....com>
To:	Pekka J Enberg <penberg@...helsinki.fi>
cc:	Matthew Wilcox <matthew@....cx>, akpm@...ux-foundation.org,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [patch 09/10] SLUB: Do our own locking via slab_lock and
 slab_unlock.

On Sun, 28 Oct 2007, Pekka J Enberg wrote:

> It would be easier to review the actual locking changes if you did the 
> SlabXXX removal in a separate patch.

There are no locking changes.

> > -static __always_inline void slab_lock(struct page *page)
> > +static __always_inline void slab_unlock(struct page *page,
> > +					unsigned long state)
> >  {
> > -	bit_spin_lock(PG_locked, &page->flags);
> > +	smp_wmb();
> 
> Memory barriers deserve a comment. I suppose this is protecting 
> page->flags but against what?

Its making sure that the changes to page flags are made visible after all 
other changes.

> 
> > +	page->flags = state;
> > +	preempt_enable();
> 
> We don't need preempt_enable for CONFIG_SMP, right?

preempt_enable is needed if preemption is enabled.

> 
> > +	 __release(bitlock);
> 
> This needs a less generic name and maybe a comment explaining that it's 
> not annotating a proper lock? Or maybe we can drop it completely?

Probably.

> > +static __always_inline unsigned long slab_trylock(struct page *page)
> > +{
> > +	unsigned long state;
> > +
> > +	preempt_disable();
> > +	state = page->flags & ~LOCKED;
> > +#ifdef CONFIG_SMP
> > +	if (cmpxchg(&page->flags, state, state | LOCKED) != state) {
> > +		 preempt_enable();
> > +		 return 0;
> > +	}
> > +#endif
> 
> This is hairy. Perhaps it would be cleaner to have totally separate 
> functions for SMP and UP instead?

I think that is reasonably clear. Having code duplicated is not good 
either. Well we may have to clean this up a bit.



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