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

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Date:   Wed, 28 Aug 2019 11:27:39 +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 2019-08-27 08:53:06 [-0700], Paul E. McKenney wrote:
> > > On the other hand, within a PREEMPT=n kernel, the call to schedule()
> > > would split even an rcu_read_lock() critical section.  Which is why I
> > > asked earlier if sleeping_lock_inc() and sleeping_lock_dec() are no-ops
> > > in !PREEMPT_RT_BASE kernels.  We would after all want the usual lockdep
> > > complaints in that case.
> > 
> > sleeping_lock_inc() +dec() is only RT specific. It is part of RT's
> > spin_lock() implementation and used by RCU (rcu_note_context_switch())
> > to not complain if invoked within a critical section.
> 
> Then this is being called when we have something like this, correct?
> 
> 	DEFINE_SPINLOCK(mylock); // As opposed to DEFINE_RAW_SPINLOCK().
> 
> 	...
> 
> 	rcu_read_lock();
> 	do_something();
> 	spin_lock(&mylock); // Can block in -rt, thus needs sleeping_lock_inc()
> 	...
> 	rcu_read_unlock();
> 
> Without sleeping_lock_inc(), lockdep would complain about a voluntary
> schedule within an RCU read-side critical section.  But in -rt, voluntary
> schedules due to sleeping on a "spinlock" are OK.
> 
> Am I understanding this correctly?

Everything perfect except that it is not lockdep complaining but the
WARN_ON_ONCE() in rcu_note_context_switch().

> 
> 							Thanx, Paul

Sebastian