[PATCH 0/3] coupled cpuidle state support

[Colin Cross]

On Wed, Dec 21, 2011 at 1:44 AM, Arjan van de Ven <arjan at linux.intel.com> wrote:
> On 12/21/2011 10:40 AM, Colin Cross wrote:
>
>>> this smells fundamentally racey to me; you can get an interrupt one
>>> cycle after you think you're done, but before the last guy enters WFI...
>>>
>>> how do you solve that issue ?
>>
>> All the cpus have interrupts off when they increment the counter, so
>> they cannot receive an interrupt.  If an interrupt is pending on one
>> of those cpus, it will be handled later when WFI aborts due to the
>> pending interrupt.
>
> ... but this leads to cases where you're aborting before other cpus are
> entering..... so your "last guy in" doesn't really work, since while cpu
> 0 thinks it's the last guy, cpu 1 is already on the way out/out
> already...  (heck it might already be going back to sleep if your idle
> code can run fast, like in the size of a cache miss)

Once a cpu has incremented the counter, it has no way out unless either
1: another cpu (that hasn't incremented the counter yet) receives an
interrupt, aborts idle, and clears its idle flag
or
2: all cpus enter the ready counter, and call the cpuidle driver's
enter function.

In your example, cpu 1 has incremented the counter, so it cannot be on
the way out unless cpu 0 aborts (in which case it will not increment
the counter, and the counter will never be equal to the number of
cpus), or unless cpu 0 turns off its interrupts and incrementes the
counter (in which case neither cpu can return until after the cpuidle
driver's enter function has been called on all cpus).