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

Pekka J Enberg
2007-10-28T15:10:49Z

Hi Christoph,

On Sat, 27 Oct 2007, Christoph Lameter wrote:
> Too many troubles with the bitlocks and we really do not need
> to do any bitops. Bitops do not effectively retrieve the old
> value which we want. So use a cmpxchg instead on the arches
> that allow it.

> -static inline int SlabFrozen(struct page *page)
> -{
> - return page->flags & FROZEN;
> -}
> -
> -static inline void SetSlabFrozen(struct page *page)
> -{
> - page->flags |= FROZEN;
> -}

[snip]

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

> +#ifdef __HAVE_ARCH_CMPXCHG
> /*
> * Per slab locking using the pagelock
> */
> -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?

> + page->flags = state;
> + preempt_enable();

We don't need preempt_enable for CONFIG_SMP, right?

> + __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?

> +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?

> -static __always_inline void slab_unlock(struct page *page)
> +static __always_inline unsigned long slab_lock(struct page *page)
> {
> - bit_spin_unlock(PG_locked, &page->flags);
> + unsigned long state;
> +
> + preempt_disable();
> +#ifdef CONFIG_SMP
> + do {
> + state = page->flags & ~LOCKED;
> + } while (cmpxchg(&page->flags, state, state | LOCKED) != state);
> +#else
> + state = page->flags & ~LOCKED;
> +#endif

Same here.

Pekka