linux-kernel - Re: Random panic in load

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Message-ID: <CA+55aFzDWritxv5RiqwcZ1qCKJQKK-Lw6StYFF7qx5YWgiCUTQ@mail.gmail.com>
Date:	Wed, 23 Jul 2014 10:26:21 -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:
>
> sched_init() definitely does _not_ allocate a cpumask_var.

Side note: another good rule of thumb for per-cpu variables is:

 - if you use __get_cpu_var() without taking the address of it, you're
doing something wrong and stupid.

The whole - and really *only* - point of __get_cpu_var is to get the
address of a a cpu variable. If you want to read the *value* of the
variable, you should use "this_cpu_read()", which can use things like
special instructions or segments to read the percpu area.

I agree that the interface is not all that great, there's historical
baggage there. We would have been better off with
"__this_cpu_ptr(var)" instead of "&__get_cpu_var(var)". But that
"__get_cpu_var()" is the old way of doing things (predating the new
and better "this_cpu_read/write/ops()" stuff), which is why we have
that odd interface with "&__get_cpu_var()".

           Linus
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