Re: [PATCH RFC tip/core/rcu 12/15] lib/assoc_array: Remove smp_read_barrier_depends()

On Wed, Oct 11, 2017 at 06:12:20PM +0200, Peter Zijlstra wrote:

On Wed, Oct 11, 2017 at 08:59:48AM -0700, Paul E. McKenney wrote:

quoted

On Wed, Oct 11, 2017 at 04:17:25PM +0100, David Howells wrote:

quoted

I will, however, quibble with the appropriateness of the name READ_ONCE()...
I still think it's not sufficiently obvious that this is a barrier and the
barrier is after.  Maybe READ_AND_BARRIER()?

Linus was unhappy with READ_ONCE_CTRL() to tag control dependencies, but
indicated that he might consider it if it helped code-analysis tools.
Adding Dmitry Vyukov for his thoughts on whether tagging READ_ONCE()
for dependencies would help.  Me, I would suggest READ_ONCE_DEP(), but
let's figure out if the bikeshed needs to be painted before arguing over
the color.  ;-)

Count me one vote for the READ_ONCE() name. This is about dependent
reads, which are nothing special on anything except Alpha.

We want to remove the exception/specialness from the memory model; and
therefore have to fix up all primitives that could possibly be used for
these reads to unconditionally issue the barrier (on Alpha). The
alternative is: rm -rf arch/alpha.

Adding something like READ_ONCE_DEP() does not rid us of the idea that
dependent reads are special and thus defeats the purpose, we might as
well retain lockless_dereference().

Now; any user of dependent reads must use READ_ONCE() in any case, to
avoid load tearing and reloads. So using READ_ONCE() for the dependent
reads is not extra or additional (note we'll also have to add the
barrier to all our relaxed and release atomics and anything else that
implies READ_ONCE and doesn't already imply smp_mb() after).

Add the per-cpu ops to that list, they imply READ_ONCE(). Consider for
example this example:


	for_each_possible_cpu(cpu)
		smp_store_release(per_cpu_ptr(&foo, cpu), obj);

-vs-

	obj = this_cpu_read(foo);
	if (obj->ponies)
		fart_rainbow(obj);