linux-kernel - Re: Random panic in load

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Message-ID: <CA+55aFz9z=58iEj3qx4Oip=iPYxzgSaPe945Xzd8Z60_v1EEzQ@mail.gmail.com>
Date:	Wed, 23 Jul 2014 11:07:40 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Dietmar Eggemann <dietmar.eggemann@....com>,
	Michel Dänzer <michel@...nzer.net>,
	Ingo Molnar <mingo@...hat.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: Random panic in load_balance() with 3.16-rc

On Wed, Jul 23, 2014 at 10:12 AM, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
> A cpumask_var is TOTALLY DIFFERENT. It's *either* a cpumask _or_ just
> a pointer to an externally allocated cpumask.
>
> sched_init() definitely does _not_ allocate a cpumask_var.

I take that back. It does end up allocating it properly, it just
avoids all the correct abstractions.

In general, the rule of thumb should be:

 - stack allocations should use "cpumask_var_t cpus" and they
absolutely *have* to be paired with an "alloc_cpumask_var(&cpus,
GFP_KERNEL)". Having a "struct cpumask" on stack is very wrong.

 - random single nonstack allocations should probably just use a plain
"struct cpumask" (or cpumask_t, but we really shouldn't use typedef's
unless they actively abstract some per-config *changing* type).

 - dynamic allocations that are size-conscious (because there's a lot
of them) should allocate a "struct cpumask *" by using
"cpumask_size()". They have a pointer anyway, they allocate things
dynamically anyway, extra indirection through a cpumask_var_t would
just be unnecessary.

 - *static* per-cpu allocations might want to use "cpumask_var_t" (to
avoid having a full "struct cpumask_t") along with doing a
"zalloc_cpumask_var_node(..)" for each cpu.

sched_init() follows that last pattern, except it open-codes that
zalloc_cpumask_var_node() in an odd way that confused me.

So I take my patch back. It's wrong, because it only allocates that
cpumask_size() if CONFIG_CPUMASK_OFFSTACK is true.

Ugh, that code really is unreadable.

               Linus
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