Re: [patch 3/3] mm: PageActive no testset

[Nick Piggin]

Hi Marcelo,

On Wed, Jan 18, 2006 at 12:13:46PM -0200, Marcelo Tosatti wrote:
> Hi Nick,
> 
> On Wed, Jan 18, 2006 at 11:40:58AM +0100, Nick Piggin wrote:
> > PG_active is protected by zone->lru_lock, it does not need TestSet/TestClear
> > operations.
> 
> page->flags bits (including PG_active and PG_lru bits) are touched by
> several codepaths which do not hold zone->lru_lock. 
> 
> AFAICT zone->lru_lock guards access to the LRU list, and no more than
> that.
> 

Yep.

> Moreover, what about consistency of the rest of page->flags bits?
> 

That's OK, set_bit and clear_bit are atomic as well, they just don't
imply memory barriers and can be implemented a bit more simply.

The test-set / test-clear operations also kind of imply that it is
being used for locking or without other synchronisation (usually).

> PPC for example implements test_and_set_bit() with:
> 
> 	lwarx	reg, addr   (load and create reservation for 32-bit addr)
> 	or 	reg, BITOP_MASK(nr)	
> 	stwcx	reg, addr  (store word upon reservation validation, otherwise loop)
> 

Thanks,
Nick
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