how are callouts handled in cpu_idle() ?

[Nathan Whitehorn]

On 10/01/11 04:25, Alexander Motin wrote:
> Hi.
>
> Adrian Chadd wrote:
>> What happens if this occurs:
>>
>> * cpu_idle() is entered
>> * you enter critical_enter() and call cpu_idleclock()
>> * an interrupt occurs, scheduling an immediate callout
>> * .. but you've already set the clock register, so it won't be
>> serviced until the wait returns.
>>
>> Perhaps interrupts have to be disabled before critical_enter() and
>> cpu_idletick() to ensure an interrupt-driven callout doesn't get
>> delayed?
> Use of critical section in cpu_idle() from the beginning was based on
> number of assumptions about filter interrupt handler's limitations.
> These handlers are not guarantied to get updated system time/ticks and
> they are discouraged to use callouts. If callout scheduled from
> interrupt filter during system wake up, system has no ticks counter
> updated yet and you may get callout scheduled into the past (it will be
> run immediately) or at least much earlier (up to 250ms) then requested.
> In your case callout indeed may get delayed (up to the same 250ms). All
> that is not a problem for regular interrupt threaded interrupts --
> interrupt thread execution will be delayed until all stuff get updated.
>

Can you explain why the critical section is there in more detail? It 
seems like all of our problems arise because of it.
-Nathan