Re: x86 ptep_get_and_clear question

[Jamie Lokier <lk@tantalophile.demon.co.uk>]

Manfred Spraul wrote:
> >         entry = ptep_get_and_clear(pte);
> >         set_pte(pte, pte_modify(entry, newprot));
> > 
> > I.e. the only code with the race condition is code which explicitly
> > clears the dirty bit, in vmscan.c.
> > 
> > Do you see any possibility of losing a dirty bit here?
> >
> Of course.
> Just check the output after preprocessing.
> It's 
> 	int entry;
> 	entry = *pte;
> 	entry &= ~_PAGE_CHG_MASK;
> 	entry |= pgprot_val(newprot)
> 	*pte = entry;

And how does that lose a dirty bit?

For the other processor to not write a dirty bit, it must have a dirty
TLB entry already which, along with the locked cycle in
ptep_get_and_clear, means that `entry' will have _PAGE_DIRTY set.  The
dirty bit is not lost.

> We need
> 	atomic_clear_mask (_PAGE_CHG_MASK, pte);
> 	atomic_set_mask (pgprot_val(newprot), *pte);
> 
> for multi threaded apps.

cmpxchg is probably faster.

-- Jamie