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

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Message-ID: <87y29ipqug.ffs@tglx>
Date:   Tue, 03 Aug 2021 16:04:07 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Peter Zijlstra <peterz@...radead.org>,
        Mike Galbraith <efault@....de>
Cc:     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 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?

Thanks,

        tglx