On Wed, 25 Apr 2007, Christoph Lameter wrote:
> On Wed, 25 Apr 2007, Hugh Dickins wrote:
>
> > > Right. local_irq_save does not switch off preemption as I thought.
> >
> > Strange comment. Preemption is not possible while IRQs are disabled,
> > but new_slab() rightly reenables them within itself in the __GFP_WAIT
> > case, since it's going off to do a page allocation and may need to wait.
>
> Yes I expected local_irq_save to increase the preempt count and then
> local_irq_enable to simply enable interrupts without affecting the preempt
> count. Thus the process should stay on the same processor.
>
> Never thought it would be possible to move to a different processor in mid
> flight.
But, surely you wouldn't have expected it to stay on the processor
throughout the waiting page allocation?? I think you're misremembering
your expectations, and this was just a simple, understandable, oversight.
Quite a serious one, though: it got caught in my case by the NULL
dereference, but it's probably been switching cpu there much more
often - one cpu diddling with what's private to another, with
unpredictable results.
Hugh
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