Do per cpu variables need to be atomic?

[Christoph Lameter]

On Thu, 10 Jul 2014, Pranith Kumar wrote:

> Hello,
>
> Let us say we have a per-cpu data structure as follows:
>
> struct data {
>         atomic_t val;
> };
>
> Since this is a per-cpu data structure, do I need to have 'val' as
> atomic? Or can I just use a normal 'int' for val?

Per cpu data structures are for the use of one cpu exclusively. If you use
the variable as intended then there is no atomic_t required. If you want
to update the percpu variable from other processors then atomic_t is
required. But then this is not a proper percpu variable anymroe.


> > Also are the following the same? If yes, which is preferable? >
> DEFINE_PER_CPU(struct data, datap);

datap is now a fixed offset into the percpu area. The processor can encode
a load using a segment prefix and the fixed offset if the this_cpu
operations are being used for accesses.


> struct data __percpu *p = &datap;

This wont work without the definition above and will assign the offset of
datap in the percpu area to p. p can be used with this_cpu operations but
is not a generally usable pointer. Its an offset into the per cpu area.

> struct data *p = this_cpu_ptr(&datap);

This converts the offset into an address that can be generally used as
a pointer (but no longer with this_cpu operations).