Re: Random panic in load_balance() with 3.16-rc

Linus Torvalds
2014-07-23T17:26:24Z

On Wed, Jul 23, 2014 at 10:12 AM, Linus Torvalds
<torvalds@linux-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