Re: [PATCH V4 1/3] cpufreq: Make sure frequency transitions are serialized

[Catalin Marinas <catalin.marinas@arm.com>]

On Fri, Mar 21, 2014 at 11:24:16AM +0000, Srivatsa S. Bhat wrote:
> On 03/21/2014 04:35 PM, Catalin Marinas wrote:
> > On Fri, Mar 21, 2014 at 09:21:02AM +0000, Viresh Kumar wrote:
> >> @Catalin: We have a problem here and need your expert advice. After changing
> >> CPU frequency we need to call this code:
> >>
> >> cpufreq_notify_post_transition();
> >> policy->transition_ongoing = false;
> >>
> >> And the sequence must be like this only. Is this guaranteed without any
> >> memory barriers? cpufreq_notify_post_transition() isn't touching
> >> transition_ongoing at all..
> > 
> > The above sequence doesn't say much. As rmk said, the compiler wouldn't
> > reorder the transition_ongoing write before the function call. I think
> > most architectures (not sure about Alpha) don't do speculative stores,
> > so hardware wouldn't reorder them either. However, other stores inside
> > the cpufreq_notify_post_transition() could be reordered after
> > transition_ongoing store. The same for memory accesses after the
> > transition_ongoing update, they could be reordered before.
> > 
> > So what we actually need to know is what are the other relevant memory
> > accesses that require strict ordering with transition_ongoing.
> 
> Hmm.. The thing is, _everything_ inside the post_transition() function
> should complete before writing to transition_ongoing. Because, setting the
> flag to 'false' indicates the end of the critical section, and the next
> contending task can enter the critical section.

smp_mb() is all about relative ordering. So if you want memory accesses
in post_transition() to be visible to other observers before
transition_ongoing = false, you also need to make sure that the readers
of transition_ongoing have a barrier before subsequent memory accesses.

> > What I find strange in your patch is that
> > cpufreq_freq_transition_begin() uses spinlocks around transition_ongoing
> > update but cpufreq_freq_transition_end() doesn't.
> 
> The reason is that, by the time we drop the spinlock, we would have set
> the transition_ongoing flag to true, which prevents any other task from
> entering the critical section. Hence, when we call the _end() function,
> we are 100% sure that only one task is executing it. Hence locks are not
> necessary around that second update. In fact, that very update marks the
> end of the critical section (which acts much like a spin_unlock(&lock)
> in a "regular" critical section).

OK, I start to get it. Is there a risk of missing a wake_up event? E.g.
one thread waking up earlier, noticing that transition is in progress
and waiting indefinitely?

-- 
Catalin