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

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Date:   Wed, 28 Aug 2019 15:14:33 +0200
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     "Paul E. McKenney" <paulmck@...nel.org>
Cc:     Joel Fernandes <joel@...lfernandes.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 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.

> 							Thanx, Paul

Sebastian