On Fri, Jul 24, 2015 at 08:29:05AM -0700, Paul E. McKenney wrote:
> Hello, Peter,
>
> The ring-buffer code uses control dependencies, and the shiny new
> READ_ONCE_CTRL() is now in mainline. I was idly curious about whether
> the write side could use smp_store_release(), and I found this:
>
> static inline void perf_mmap__write_tail(struct perf_mmap *md, u64 tail)
> {
> struct perf_event_mmap_page *pc = md->base;
>
> /*
> * ensure all reads are done before we write the tail out.
> */
> mb();
> pc->data_tail = tail;
> }
>
> I see mb() rather than smp_mb(). Did I find the correct code for the
> write side? If so, why mb() rather than smp_mb()? To serialize against
> MMIO interactions with hardware counters or some such?
This is userspace, it doesn't patch itself depending on if its run on an
SMP machine or not.
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