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

Peter Zijlstra
2009-02-16T21:04:14Z

On Mon, 2009-02-16 at 21:49 +0100, Oleg Nesterov wrote:
> 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?

I was about to write a response, but found it to be a justification for
the read_barrier_depends() at the end of the loop.

> 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 ?

Would it not be the caller's responsibility to provide the needed
serialization in this case?