Peter Zijlstra <pet...@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
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majord...@vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/
