perf_mmap__write_tail() and control dependencies

[Paul E. McKenney]

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?

							Thanx, Paul

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