Re: Possible memory ordering bug in page reclaim?

From: Andrea Arcangeli
Date: Sat Oct 15 2005 - 15:09:26 EST

On Sun, Oct 16, 2005 at 05:48:55AM +1000, Herbert Xu wrote:
> On Sat, Oct 15, 2005 at 06:00:18PM +0000, Andrea Arcangeli wrote:
> >
> > Note that the barrier in atomic_add_negative is useless here because it
> > happens way too late, _after_ the count is decremented (not _before_)
> > so the decreased count could be already visible to the other cpu.
> Could you please point me to an architecture that does this?

sure see alpha:

__asm__ __volatile__(
"1: ldq_l %0,%1\n"
" addq %0,%3,%2\n"
" addq %0,%3,%0\n"
" stq_c %0,%1\n"
" beq %0,2f\n"
" mb\n"

the memory barrier is applied way after the write is visible to other
cpus, you can even get an irq before the mb and block there for some

>From a common code point of view, the barrier you mentioned in
atomic_add_negative is absolutely useless for this specific case
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