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

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Message-ID: <Pine.LNX.4.64.0710281702140.6766@sbz-30.cs.Helsinki.FI>
Date:	Sun, 28 Oct 2007 17:10:36 +0200 (EET)
From:	Pekka J Enberg <penberg@...helsinki.fi>
To:	Christoph Lameter <clameter@....com>
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.

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
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