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).
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