On Wed, 16 May 2007 16:40:59 -0700 (PDT) Christoph Lameter <[email protected]> wrote:
> On Wed, 16 May 2007, Andrew Morton wrote:
>
> > (I hope. Might have race windows in which the percpu_counter_sum() count is
> > inaccurate?)
>
> The question is how do these race windows affect the locking scheme?
The race to which I refer here is if another CPU is running
percpu_counter_sum() in the window between the clearing of the bit in
cpu_online_map and the CPU_DEAD callout. Maybe that's too small to care
about in the short-term, dunno.
Officially we should fix that by taking lock_cpu_hotplug() in
percpu_counter_sum(), but I hate that thing.
I was thinking of putting a cpumask into the counter. If we do that then
there's no race at all: everything happens under fbc->lock. This would be
a preferable fix, if we need to fix it.
But I'd prefer that freezer-based cpu-hotplug comes along and saves us
again.
umm, actually, we can fix the race by using CPU_DOWN_PREPARE instead of
CPU_DEAD. Because it's OK if percpu_counter_sum() looks at a gone-away
CPU's slot.
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