On Wed, 15 Dec 2010, Matthew Wilcox wrote:
> 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.
Very helpful. Thanks!
julia
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