Re: [patch 09/10] SLUB: Do our own locking via slab_lock and slab_unlock.

Christoph Lameter
2007-10-29T03:03:21Z

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.