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SubjectRe: [PATCH V5] powercap/drivers/idle_injection: Add an idle injection framework
Hi Daniel, Viresh,

On Wed, Jun 06, 2018 at 04:15:28PM +0530, Viresh Kumar wrote:
> On 06-06-18, 12:22, Daniel Lezcano wrote:
> > (mb() are done in the atomic operations AFAICT).

To do my bit, not all atomic ops do/imply memory barriers; e.g.,

[from Documentation/atomic_t.txt]

- non-RMW operations [e.g., atomic_set()] are unordered

- RMW operations that have no return value [e.g., atomic_inc()] are unordered


>
> AFAIU, it is required to make sure the operations are seen in a particular order
> on another CPU and the compiler doesn't reorganize code to optimize it.
>
> For example, in our case what if the compiler reorganizes the atomic-set
> operation after wakeup-process ? But maybe that wouldn't happen across function
> calls and we should be safe then.

IIUC, wake_up_process() implies a full memory barrier and a compiler barrier,
due to:

raw_spin_lock_irqsave(&p->pi_lock, flags);
smp_mb__after_spinlock();

The pattern under discussion isn't clear to me, but if you'll end up relying
on this "implicit" barrier I'd suggest documenting it with a comment.

Andrea

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