linux-kernel - Re: [ANNOUNCE] v5.14-rc4-rt4

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Date:   Wed, 4 Aug 2021 12:17:04 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Jens Axboe <axboe@...nel.dk>
Cc:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Daniel Wagner <wagi@...om.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-rt-users@...r.kernel.org
Subject: Re: [ANNOUNCE] v5.14-rc4-rt4

On Wed, 4 Aug 2021 09:49:48 -0600
Jens Axboe <axboe@...nel.dk> wrote:

> > @@ -430,9 +430,9 @@ static struct io_wq_work *io_get_next_work(struct io_wqe *wqe)
> >  	}
> >  
> >  	if (stall_hash != -1U) {
> > -		raw_spin_unlock(&wqe->lock);
> > +		raw_spin_unlock_irq(&wqe->lock);
> >  		io_wait_on_hash(wqe, stall_hash);
> > -		raw_spin_lock(&wqe->lock);
> > +		raw_spin_lock_irq(&wqe->lock);
> >  	}
> >  
> >  	return NULL;
> > 
> > (this is on-top of the patch you sent earlier and Daniel Cc: me on after
> > I checked that the problem/warning still exists).  
> 
> That'd work on non-RT as well, but it makes it worse on non-RT as well with
> the irq enable/disable dance. While that's not the end of the world, would
> be nice to have a solution that doesn't sacrifice anything, yet doesn't
> make RT unhappy.

We use to have something like:

	local_irq_disable_rt()

that would only disable irqs when PREEMPT_RT was configured, but this
was considered "ugly" and removed to try to only use spin_lock_irq()
and raw_spin_lock_irq(). But for this situation, it looks like it would
do exactly what you wanted. Not sacrifice anything yet keep RT happy.

Not sure that's a solution still. :-/

Perhaps in this situation, we could open code it to:

  	if (stall_hash != -1U) {
		raw_spin_unlock(&wqe->lock);

		/* On RT the spin_lock_irq() does not disable interrupts */
		if (IS_ENABLED(CONFIG_PREEMPT_RT))
			local_irq_enable();

 		io_wait_on_hash(wqe, stall_hash);

		if (IS_ENABLED(CONFIG_PREEMPT_RT))
			local_irq_disable();
		raw_spin_lock(&wqe->lock);
 	}

Note, I haven't looked at the rest of the code to know the ripple
effect of this, but I'm just suggesting the idea.

-- Steve