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

[Viresh Kumar]

On 21 March 2014 14:51, Viresh Kumar <viresh.kumar@xxxxxxxxxx> 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..

For others this is what we discussed on IRC (rmk: Russell King)

<rmk> I'm no barrier expert, but the compiler can't reorder that assignment
across a function call which it knows nothing about (which it can't
know anything
about because it calls other functions through function pointers)

<rmk> however, the CPU could re-order the effects with respect to other agents
(cpus/devices) when they look at the memory

<rmk> for the local CPU, the question is really: what does the C
language virtual
machine say about this - that's what really matters.  If the CPU does
speculative
stuff, it still has to make the machine behaviour fit that model.
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