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

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Message-ID: <20210803094832.GA8057@worktop.programming.kicks-ass.net>
Date:   Tue, 3 Aug 2021 11:48:32 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Mike Galbraith <efault@....de>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Will Deacon <will@...nel.org>,
        Waiman Long <longman@...hat.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Davidlohr Bueso <dave@...olabs.net>
Subject: Re: [patch 03/63] sched: Prepare for RT sleeping spin/rwlocks

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);