Re: perf_mmap__write_tail() and control dependencies

[Peter Zijlstra <peterz@infradead.org>]

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.