Q: smp.c && barriers (Was: [PATCH 1/4] generic-smp: remove single ipi fallback for smp_call_function_many())

[Oleg Nesterov <oleg@redhat.com>]

I am trying to understand the barriers in smp.c, please help!

"generic-ipi: fix the smp_mb() placement" commit
561920a0d2bb6d63343e83acfd784c0a77bd28d1 added smp_read_barrier_depends()
to generic_smp_call_function_single_interrupt().

Why it is needed? The comment says:

	/*
	 * Need to see other stores to list head for checking whether
	 * list is empty without holding q->lock
	 */
	smp_read_barrier_depends();
	while (!list_empty(&q->list)) {

But we can't miss the addition to the call_single_queue.list,
if generic_exec_single() sees list_empty(&dst->list) it sends
another IPI?


This commit also removed the barrier from csd_flag_wait(), is this OK?
Without the barrier, csd_flag_wait() can return before we see the result
of data->func() ?

IOW,
	int VAR = 0;

	void func(coid *unused)
	{
		VAR = 1;
	}

Now,

	smp_call_function_single(0, func, NULL, 1);
	BUG_ON(VAR == 0);

afaics, the BUG_ON() above is possible. Is this OK ?

Oleg.