On 10/28, Paul E. McKenney wrote:

On Mon, Oct 28, 2013 at 02:26:34PM +0100, Peter Zijlstra wrote:

quoted

--- linux-2.6.orig/kernel/events/ring_buffer.c
+++ linux-2.6/kernel/events/ring_buffer.c

@@ -87,10 +87,31 @@ static void perf_output_put_handle(struc
 		goto out;

 	/*
-	 * Publish the known good head. Rely on the full barrier implied
-	 * by atomic_dec_and_test() order the rb->head read and this
-	 * write.
+	 * Since the mmap() consumer (userspace) can run on a different CPU:
+	 *
+	 *   kernel				user
+	 *
+	 *   READ ->data_tail			READ ->data_head
+	 *   smp_rmb()	(A)			smp_rmb()	(C)

Given that both of the kernel's subsequent operations are stores/writes,
doesn't (A) need to be smp_mb()?

Yes, this is my understanding^Wfeeling too, but I have to admit that
I can't really explain to myself why _exactly_ we need mb() here.

And let me copy-and-paste the artificial example from my previous
email,

	bool	BUSY;
	data_t 	DATA;

	bool try_to_get(data_t *data)
	{
		if (!BUSY)
			return false;

		rmb();

		*data = DATA;
		mb();
		BUSY = false;

		return true;
	}

	bool try_to_put(data_t *data)
	{
		if (BUSY)
			return false;

		mb();	// XXXXXXXX: do we really need it? I think yes.

		DATA = *data;
		wmb();
		BUSY = true;

		return true;
	}

(just in case, the code above obviously assumes that _get or _put
 can't race with itself, but they can race with each other).

Could you confirm that try_to_put() actually needs mb() between
LOAD(BUSY) and STORE(DATA) ?

I am sure it actually needs, but I will appreciate it if you can
explain why. IOW, how it is possible that without mb() try_to_put()
can overwrite DATA before try_to_get() completes its "*data = DATA"
in this particular case.

Perhaps this can happen if, say, reader and writer share a level of
cache or something like this...

Oleg.