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

[Peter Z. <a.p...@ch...>]

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);
}

static inline
int chunk_head_wait(struct chunk_head *ch)
{
	while (test_bit(PG_locked, &ch->flags))
		relax_cpu();
	smp_rmb();
}