Re: [lc-devel] Announce: Compressed cache alpha-007 (kernel 2.6.18)

[Nick P. <np...@su...>]

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