Re: pserialize(9) vs. TAILQ

[Taylor R Campbell <campbell+netbsd-tech-kern%mumble.net@localhost>]

   Date: Mon, 24 Nov 2014 18:36:43 +0100
   From: Rhialto <rhialto%falu.nl@localhost>

   I must admit that it is not clear to me how on all these architectures
   the CPU is to know in general that *p depends on p = *pp.

Issuing membar_producer on CPU 1 may flush its store buffer to RAM and
force all other CPUs to discard their caches for everything in its
store buffer.

Then when CPU 2 executes p = *pp, if it sees the new version of *pp,
it must have discarded its cache for *p, so when it executes v = *p,
it will see the value of *p that CPU 1 put in there before assigning
*pp = p.

That doesn't address control-dependent loads, which is why even CPUs
that do the above may require membar_consumer for control-dependent
loads.  Specifically, if CPU 1 does

	value[i] = 5;
	membar_producer();
	ok[i] = 1;

then because there's no data dependency, CPU 2 might still reorder

	if (ok[i])
		v = value[i];

into

	tmp = value[i];
	if (ok[i])
		v = tmp;

just by executing instructions out of order, with no cache involved.