linux-kernel - Re: [PATCH RT v2 2/3] sched: migrate_enable: Use sleeping

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Date:   Wed, 28 Aug 2019 09:59:38 -0400
From:   Joel Fernandes <joel@...lfernandes.org>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:     "Paul E. McKenney" <paulmck@...nel.org>,
        Scott Wood <swood@...hat.com>, linux-rt-users@...r.kernel.org,
        linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Clark Williams <williams@...hat.com>
Subject: Re: [PATCH RT v2 2/3] sched: migrate_enable: Use sleeping_lock to
 indicate involuntary sleep

On Wed, Aug 28, 2019 at 03:14:33PM +0200, Sebastian Andrzej Siewior wrote:
> On 2019-08-28 05:54:26 [-0700], Paul E. McKenney wrote:
> > On Wed, Aug 28, 2019 at 11:27:39AM +0200, Sebastian Andrzej Siewior wrote:
> > > On 2019-08-27 08:53:06 [-0700], Paul E. McKenney wrote:
> > > > Am I understanding this correctly?
> > > 
> > > Everything perfect except that it is not lockdep complaining but the
> > > WARN_ON_ONCE() in rcu_note_context_switch().
> > 
> > This one, right?
> > 
> > 	WARN_ON_ONCE(!preempt && t->rcu_read_lock_nesting > 0);
> > 
> > Another approach would be to change that WARN_ON_ONCE().  This fix might
> > be too extreme, as it would suppress other issues:
> > 
> > 	WARN_ON_ONCE(IS_ENABLED(CONFIG_PREEMPT_RT_BASE) && !preempt && t->rcu_read_lock_nesting > 0);
> > 
> > But maybe what is happening under the covers is that preempt is being
> > set when sleeping on a spinlock.  Is that the case?
> 
> I would like to keep that check and that is why we have:
> 
> |   #if defined(CONFIG_PREEMPT_RT_FULL)
> |         sleeping_l = t->sleeping_lock;
> |   #endif
> |         WARN_ON_ONCE(!preempt && t->rcu_read_lock_nesting > 0 && !sleeping_l);
> 
> in -RT and ->sleeping_lock is that counter that is incremented in
> spin_lock(). And the only reason why sleeping_lock_inc() was used in the
> patch was to disable _this_ warning.

Makes sense, Sebastian.

Paul, you meant "!" in front of the IS_ENABLED right in your code snippet right?

The other issue with:
WARN_ON_ONCE(!IS_ENABLED(CONFIG_PREEMPT_RT_BASE) && !preempt && t->rcu_read_lock_nesting > 0);

.. could be that, the warning will be disabled for -rt entirely, not just for
sleeping locks. And we probably do want to keep this warning for the cases in
-rt where we are blocking but it is not a sleeping lock. Right?

thanks,

 - Joel