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From: Rusty Russell <rusty@rustcorp.com.au>
To: Eric Dumazet <eric.dumazet@gmail.com>
Subject: Re: [RFC PATCH] x86-64: software IRQ masking and handling
Date: Mon, 12 Jul 2010 17:32:24 +0930
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On Mon, 12 Jul 2010 02:41:33 pm Eric Dumazet wrote:
> Two changes are possible :
> 
> 1) Get rid of the cpu_online_mask (its a const pointer to a known
> target). I cant see a reason for its need it actually...

There was a reason, but I'm trying to remember it.

ISTR, it was to catch direct frobbing of the masks.  That was important:
we were converting code everywhere to hand around cpumasks by ptr
rather than by copy.  But that semantic change meant that a function which
previously harmlessly frobbed a copy would now frob (say) cpu_online_mask.

However, ((const struct cpumask *)cpu_online_bits)) would work for that
too.  (Well, renaming cpu_online_bits to __cpu_online_bits would be better
since it's not non-static).

Ideally, those masks too would be dynamically allocated.  But the boot
changes required for that are best left until someone really needs > 64k
CPUs.

> 2) Dont use a the last const qualifier but __read_mostly to move
> cpu_online_mask on same section.
> 
> Rusty, could you comment on one or other way before I submit a patch ?
> 
> (Of course, possible/present/active have same problem)

Yep.  Might want to do a patch to get rid of the remaining 100 references
to cpu_online_map (etc) as well if you're feeling enthusiastic :)

Thanks!
Rusty.