Re: [PATCHv4 17/17] writeback: lessen sync_supers wakeup count

[Nick Piggin <npiggin@suse.de>]

On Mon, May 31, 2010 at 11:25:52AM +0300, Artem Bityutskiy wrote:
> On Fri, 2010-05-28 at 01:44 +1000, Nick Piggin wrote:
> > > if (supers_dirty)
> > > 	bdi_arm_supers_timer();
> > > set_current_state(TASK_INTERRUPTIBLE);
> > > schedule();
> 
> > But we cannot do the above, because again the timer might go off
> > before we set current state. We'd lose the wakeup and never wake
> > up again.
> > 
> > Putting it inside set_current_state() should be OK. I suppose.
> 
> Hmm, but it looks like we cannot do that either. If we do
> 
> set_current_state(TASK_INTERRUPTIBLE);
> if (supers_dirty)
> 	bdi_arm_supers_timer();
> schedule();
> 
> and the kernel is preemptive, is it possible that we get preempted
> before we run 'bdi_arm_supers_timer()', but after we do
> 'set_current_state(TASK_INTERRUPTIBLE)'. And we will never wake up if
> the timer armed in mark_sb_dirty() went off.
> 
> So it looks like this is the way to go:
> 
> /*
>  * Disable preemption for a while to make sure we are not
>  * preempted before the timer is armed.
>  */
> preempt_disable();
> set_current_state(TASK_INTERRUPTIBLE);
> if (supers_dirty)
> 	bdi_arm_supers_timer();
> preempt_enable();
> schedule();

This should not be required because preempt is transparent to these
task sleep/schedule APIs.

The preempt event will not clear TASK_INTERRUPTIBLE, and so the timer
wakeup will set it to TASK_RUNNING (whether or not it has called
schedule() yet and whether or not it is currently preempted).