Re: Possible memory ordering bug in page reclaim?

[Ivan Kokshaysky]

On Sat, Oct 15, 2005 at 04:07:02PM -0700, David S. Miller wrote:
> From: Andrea Arcangeli <andrea@xxxxxxx>
> Date: Sat, 15 Oct 2005 22:07:01 +0200
> 
> > 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
> > usec.
> 
> For atomic operations returning values, there must be a memory
> barrier both before and after the atomic operation.  This is
> defined in Documentation/atomic_ops.txt, so Alpha needs to be
> fixed to add a memory barrier at the beginning of these
> assembler sequences.

My opinion is that we don't need a barrier even _after_ ll/sc sequences
on Alpha - it's redundant; perhaps it's just a historical baggage.
I strongly believe that ll/sc _does_ imply an SMP memory barrier, as can
be seen from instructions timing: a standalone mb takes tens of cycles,
the mb before/after ll/sc takes 0 to 1 cycle on ev6 (a bit more on ev56)
depending on instruction slotting.
Note that superfluous mb's around atomic stuff still can hurt -
Alpha mb instruction also flushes IO write buffers, so it can
be _extremely_ expensive under heavy IO...

Ivan.
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