perf events ring buffer memory barrier on powerpc

[Frederic Weisbecker]

2013/10/25 Peter Zijlstra <peterz at infradead.org>:
> On Wed, Oct 23, 2013 at 03:19:51PM +0100, Frederic Weisbecker wrote:
> I would argue for:
>
>   READ ->data_tail                      READ ->data_head
>   smp_rmb()     (A)                     smp_rmb()       (C)
>   WRITE $data                           READ $data
>   smp_wmb()     (B)                     smp_mb()        (D)
>   STORE ->data_head                     WRITE ->data_tail
>
> Where A pairs with D, and B pairs with C.
>
> I don't think A needs to be a full barrier because we won't in fact
> write data until we see the store from userspace. So we simply don't
> issue the data WRITE until we observe it.
>
> OTOH, D needs to be a full barrier since it separates the data READ from
> the tail WRITE.
>
> For B a WMB is sufficient since it separates two WRITEs, and for C an
> RMB is sufficient since it separates two READs.

Hmm, I need to defer on you for that, I'm not yet comfortable with
picking specific barrier flavours when both write and read are
involved in a same side :)