[linux-mm-cc] Re: Announce: Compressed cache alpha-007 (kernel 2.6.18)

[Nitin Gupta]

Nick Piggin wrote:
> On Wed, Sep 20, 2006 at 04:39:45PM +0200, Peter Zijlstra wrote:
>> On Wed, 2006-09-20 at 19:34 +0530, Nitin Gupta wrote:
>>> Peter Zijlstra wrote:
>>>> On Wed, 2006-09-20 at 18:23 +0530, Nitin Gupta wrote:
>>>>
>>>>>> Don't like the bit_spin_trylock/bit_spin_unlock in handle_ccache_fault;
>>>>>> what's wrong with TestSetPageLocked() and unlock_page() ?
>>>>>>
>>>>> If (PageCompressed(page)) is true then 'page' is 'struct chunk_head'
>>>>> not 'struct page' and you cannot use unlock_page() on chunk_head.
>>>> ClearPageLocked()
>>>>
>>> But then how will you do equivalent of wait_on_chunk_head() ?
>> Doh, now I see, its not &page->flags but &ch->flags.
>>
>> Humm, you'd need to open code this I think, bit_spin_trylock() may not
>> be ordered strong enough (on !x86). Got my head in a twist, but I think
>> this is correct:
>>
>> static inline
>> int chunk_head_trylock(struct chunk_head *ch)
>> {
>> 	int locked = test_set_bit(PG_locked, &ch->flags);
>> 	if (!locked)
>> 		smb_wmb();
>> 	return !locked;
>> }
>>
>> static inline
>> void chunk_head_unlock(struct chunk_head *ch)
>> {
>> 	int locked;
>> 	smp_wmb();
>> 	locked = test_clear_bit(PG_locked, &ch->flags);
>> 	BUG_ON(!locked);
>> }
> 
> A plain clear_bit(PG_locked, ...) here will leak, but all atomic and
> bitop RMW ops are defined to be strongly ordered before and after (by
> Documentation/atomic_ops.txt), so I think we're OK?
> 
> In your above examples, I think you probably want full smp_mb() there
> because you do want to prevent loads inside the critical section from
> being completed before the bit is visible or after the bit is cleared.
> 
> Nick
> 


I don't know much about SMP but since bit_spin_trylock() and gang is 
defined in arch independent path (include/linux/bit_spinlock.h) then 
shouldn't they be expected to provide proper locking on all archs? If 
there's known problem with them then shouldn't they be replaced with 
correct ones?

-- Nitin