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

Oleg Nesterov
2009-02-16T20:53:17Z

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.