[ltt-dev] [PATCH] Fix dirty page accounting in redirty_page_for_writepage()

[cl@linux.com (Christoph Lameter)]

On Thu, 30 Apr 2009, Mathieu Desnoyers wrote:

> > The 3 variants on x86 generate the same instructions. On other platforms
> > they would need to be able to fallback in various way depending on the
> > availability of instructions that are atomic vs. preempt or irqs.
> >
>
> The problem here, as we did figure out a while ago with the atomic
> slub we worked on a while ago, is that if we have the following code :
>
> local_irq_save
> var++
> var++
> local_irq_restore
>
> that we would like to turn into irq-safe percpu variant with this
> semantic :
>
> percpu_add_irqsafe(var)
> percpu_add_irqsafe(var)
>
> We are generating two irq save/restore in the fallback, which will be
> slow.
>
> However, we could do the following trick :
>
> percpu_irqsave(flags);
> percpu_add_irq(var);
> percpu_add_irq(var);
> percpu_irqrestore(flags);

Hmmm.I do not remember any of those double ops in the patches that I did a
while back for this. It does not make sense either because atomic per cpu
ops are only atomic for a single instruction. You are trying to extend
that so that multiple "atomic" instructions are now atomic.