Re: [PATCH RT v2 2/3] sched: migrate_enable: Use sleeping_lock to indicate involuntary sleep

From: Paul E. McKenney
Date: Mon Aug 26 2019 - 12:29:58 EST

On Mon, Aug 26, 2019 at 05:25:23PM +0200, Sebastian Andrzej Siewior wrote:
> On 2019-08-23 23:10:14 [-0400], Joel Fernandes wrote:
> > On Fri, Aug 23, 2019 at 02:28:46PM -0500, Scott Wood wrote:
> > > On Fri, 2019-08-23 at 18:20 +0200, Sebastian Andrzej Siewior wrote:
> > > >
> > > > this looks like an ugly hack. This sleeping_lock_inc() is used where we
> > > > actually hold a sleeping lock and schedule() which is okay. But this
> > > > would mean we hold a RCU lock and schedule() anyway. Is that okay?
> > >
> > > Perhaps the name should be changed, but the concept is the same -- RT-
> > > specific sleeping which should be considered involuntary for the purpose of
> > > debug checks. Voluntary sleeping is not allowed in an RCU critical section
> > > because it will break the critical section on certain flavors of RCU, but
> > > that doesn't apply to the flavor used on RT. Sleeping for a long time in an
> > > RCU critical section would also be a bad thing, but that also doesn't apply
> > > here.
> >
> > I think the name should definitely be changed. At best, it is super confusing to
> > call it "sleeping_lock" for this scenario. In fact here, you are not even
> > blocking on a lock.
> >
> > Maybe "sleeping_allowed" or some such.
> The mechanism that is used here may change in future. I just wanted to
> make sure that from RCU's side it is okay to schedule here.

Good point.

The effect from RCU's viewpoint will be to split any non-rcu_read_lock()
RCU read-side critical section at this point. This alrady happens in a
few places, for example, rcu_note_context_switch() constitutes an RCU
quiescent state despite being invoked with interrupts disabled (as is
required!). The __schedule() function just needs to understand (and does
understand) that the RCU read-side critical section that would otherwise
span that call to rcu_node_context_switch() is split in two by that call.

However, if this was instead an rcu_read_lock() critical section within
a PREEMPT=y kernel, then if a schedule() occured within stop_one_task(),
RCU would consider that critical section to be preempted. This means
that any RCU grace period that is blocked by this RCU read-side critical
section would remain blocked until stop_one_cpu() resumed, returned,
and so on until the matching rcu_read_unlock() was reached. In other
words, RCU would consider that RCU read-side critical section to span
the call to stop_one_cpu() even if stop_one_cpu() invoked schedule().

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.

Does that help, or am I missing the point?

Thanx, Paul