On Wed, Dec 15, 2010 at 12:58:49PM +0100, Julia Lawall wrote:
> I am wondering about the interaction between schedule_timeout and
> functions like local_irq_save, that turn off interrupts. To my
> understanding, with schedule_timeout one sleeps until a timer interrupt is
> received. But how can that be received if interrupts have been turned
> off? I find this mostly in older code, but there are still some
> occurrences, and even in older code, I'm not sure to understand how it
> could have worked.
My memory was that interrupts were reenabled when you called schedule(),
then restored when schedule() returned to you. That turns out not to
be true today, and I don't know whether it was ever true.
What actually happens is this:
asmlinkage void __sched schedule(void)
{
...
preempt_disable();
...
raw_spin_lock_irq(&rq->lock);
...
raw_spin_unlock_irq(&rq->lock);
...
preempt_enable_no_resched();
}
(there are two paths which lead to the rq->lock being dropped, the other one
is slightly more complex than this).
So in plain english, if you call schedule(), or schedule_timeout(),
when it returns interrupts will be enabled. It's almost certainly
a bug, and it'd be good to put a test for it in schedule(), possibly
under DEBUG_SPINLOCK_SLEEP.
--
Matthew Wilcox Intel Open Source Technology Centre
"Bill, look, we understand that you're interested in selling us this
operating system, but compare it to ours. We can't possibly take such
a retrograde step."
--
To unsubscribe from this list: send the line "unsubscribe kernel-janitors" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
