perf events ring buffer memory barrier on powerpc

[Victor Kaplansky]

Peter Zijlstra <peterz at infradead.org> wrote on 10/25/2013 07:37:49 PM:

> 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.

1. I agree. My only concern is that architectures which do use atomic
operations
with memory barriers, will issue two consecutive barriers now, which is
sub-optimal.

2. I think the comment in "include/linux/perf_event.h" describing
"data_head" and
"data_tail" for user space need an update as well. Current version -

        /*
         * Control data for the mmap() data buffer.
         *
         * User-space reading the @data_head value should issue an rmb(),
on
         * SMP capable platforms, after reading this value -- see
         * perf_event_wakeup().
         *
         * When the mapping is PROT_WRITE the @data_tail value should be
         * written by userspace to reflect the last read data. In this case
         * the kernel will not over-write unread data.
         */
        __u64   data_head;              /* head in the data section */
        __u64   data_tail;              /* user-space written tail */

- say nothing about the need of memory barrier before "data_tail" write.

-- Victor