Re: [md PATCH 04/22] md: support barrier requests on all personalities.

[Andre Noll <maan@systemlinux.org>]

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On 17:25, Neil Brown wrote:
tely Tue, 8 Dec 2009 14:54:42 +0100
> Andre Noll <maan@systemlinux.org> wrote:
> 
> Thanks again.
> I have made most of the changes you suggest.

Thanks. Feel free to add

Reviewed-by: Andre Noll <maan@systemlinux.org>

> > Calling atomic_inc() twice isn't an atomic operation any more. If
> > this doesn't matter (because all modifications of rdev->nr_pending
> > are supposed to happen within RCU read-side critical sections) then
> > why is rdev->nr_pending an atomic_t at all?
> 
> Calling atomic_inc() twice is still two atomic operations, and that is what I
> need.
> The important thing is that the read-modify-write cycle of atomic_inc isn't
> interrupted, so if two thread both do atomic_inc at the same time the result
> really is adding 2, not just adding one.

A conceivable problem is that someone else might remove the
whole thing between the two atomic_inc() calls because it calls
atomic_dec_and_test() and finds it is zero. But this can't possibly
happen here, can it?

> Multiple RCU read-side blocks can run at the same time on different
> processors - so we definitely need to protection of 'atomic_'.

Yes, definitely.

Regards
Andre
-- 
The only person who always got his work done by Friday was Robinson Crusoe

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