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

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Message-ID: <87v94m2sre.ffs@tglx>
Date:   Tue, 03 Aug 2021 22:11:17 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Mike Galbraith <efault@....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 Tue, Aug 03 2021 at 16:51, Peter Zijlstra wrote:
> On Tue, Aug 03, 2021 at 04:04:07PM +0200, Thomas Gleixner wrote:
>> On Tue, Aug 03 2021 at 11:48, Peter Zijlstra wrote:
>> 
>> > 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 :-)
>> 
>> Actually the rule to lock PI lock irqsave still persists on RT, but this
>> has to be called with interrupts disabled (rtmutex::wait_lock is held),
>> so adding a lockdep_assert_irqs_disable() might be good enough to spare
>> the extra save/restore. Hmm?
>
> Works for me.. and yes, duh, raw_.

And the lockdep_assert_irqs_disabled() is redundant as well because if
raw_spin_lock(&current->pi_lock) is invoked with interrupts enabled
lockdep will yell too. But, yes it makes it more explicit.

Thanks,

        tglx