Re: [PATCH v7 03/14] PM: Introduce an Energy Model management framework

[Quentin Perret <quentin.perret@arm.com>]

Hi Andrea,

On Tuesday 02 Oct 2018 at 21:12:17 (+0200), Andrea Parri wrote:
> An example might help clarify this: here is a scenario I can _imagine,
> based on your description.
> 
> CPU0 (em_register_perf_domain())	CPU1 (reader)
> 
> [...]					my_pd = READ_ONCE(per_cpu(em_data, 1)); /* em_cpu_get() */
> pd->table = table			if (my_pd)
> WRITE_ONCE(per_cpu(em_data, 1), pd);		my_table = my_pd->table; /* process, dereference, ... my_table */
> 
> In this scenario, we'd like CPU1 to see CPU0's store to ->table (as well
> as the stores to table[]) _if CPU1 sees CPU0's store to em_data (that is,
> if my_pd != NULL).
> 
> This guarantee does not hold with the WRITE_ONCE(), because CPU0 could
> propagate the store to ->table and the store to em_data out-of-order.

Ah, this is a very good point ... It's fine if a reader sees em_data
half way through being updated, but it is NOT fine at all if a reader
gets a pointer onto a half-baked em_perf_domain.

So I agree, there is a problem here.

(I also realize now that I totally misunderstood Peter's messages before
which were basically pointing out this issue :/ ...)

> The smp_store_release(), together with the address dependency headed by
> the READ_ONCE(), provides this guarantee (and more...).
> 
> (Enclosing the reader into an em_pd_mutex critical section would also
> provide this guarantee, but I can imagine a few arguments for not using
> a mutex... ;-) ).

Right, using the mutex in em_cpu_get() could work too, although we
probably don't want to do that. We might very well end up using it in a
RCU critical section for example (although that's not the case with this
series, I'm just looking forward a bit).

So using smp_store_release() is the best choice here. I'll fix that up
for v8.

Thank you very much,
Quentin