Re: [patch 03/63] sched: Prepare for RT sleeping spin/rwlocks

From: Peter Zijlstra
Date: Tue Aug 03 2021 - 05:49:04 EST


On Sun, Aug 01, 2021 at 05:30:06PM +0200, Mike Galbraith wrote:
> On Fri, 2021-07-30 at 15:50 +0200, Thomas Gleixner wrote:
> >
> > --- a/include/linux/sched.h
> > +++ b/include/linux/sched.h
> > @@ -155,6 +155,27 @@ struct task_group;
> >                 WRITE_ONCE(current->__state, (state_value));            \
> >                 raw_spin_unlock_irqrestore(&current->pi_lock, flags);   \
> >         } while (0)
> > +
> > +
> > +#define current_save_and_set_rtlock_wait_state()                       \
> > +       do {                                                            \
> > +               raw_spin_lock(&current->pi_lock);                       \

That wants to be irqsafe methinks, I realize this is PREEMPT_RT only and
there the _irqfoo crap is a no-op so this doesn't really matter one way
or the other, but still, taking PI lock without IRQ disable makes my
head go BUG-BUG-BUG :-)

> > +               current->saved_state = current->__state;                \
> ^^should that be using READ_ONCE()?
>

I think we're good here, per this being 'current' and serialized against
wakeup by virtue of holding pi_lock, __state is actually stable.

> > + current->saved_state_change = current->task_state_change;\
> > + current->task_state_change = _THIS_IP_; \
> > + WRITE_ONCE(current->__state, TASK_RTLOCK_WAIT); \
> > + raw_spin_unlock(&current->pi_lock); \
> > + } while (0);