Re: [PATCH rcu 3/3] rcu: Allow expedited RCU grace periods on incoming CPUs

[Paul E. McKenney]

On Wed, Feb 09, 2022 at 11:53:33PM +0530, Mukesh Ojha wrote:
> 
> On 2/5/2022 4:25 AM, Paul E. McKenney wrote:
> > Although it is usually safe to invoke synchronize_rcu_expedited() from a
> > preemption-enabled CPU-hotplug notifier, if it is invoked from a notifier
> > between CPUHP_AP_RCUTREE_ONLINE and CPUHP_AP_ACTIVE, its attempts to
> > invoke a workqueue handler will hang due to RCU waiting on a CPU that
> > the scheduler is not paying attention to.  This commit therefore expands
> > use of the existing workqueue-independent synchronize_rcu_expedited()
> > from early boot to also include CPUs that are being hotplugged.
> > 
> > Link: https://lore.kernel.org/lkml/7359f994-8aaf-3cea-f5cf-c0d3929689d6@xxxxxxxxxxx/
> > Reported-by: Mukesh Ojha <quic_mojha@xxxxxxxxxxx>
> > Cc: Tejun Heo <tj@xxxxxxxxxx>
> > Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
> > ---
> >   kernel/rcu/tree_exp.h | 14 ++++++++++----
> >   1 file changed, 10 insertions(+), 4 deletions(-)
> > 
> > diff --git a/kernel/rcu/tree_exp.h b/kernel/rcu/tree_exp.h
> > index 60197ea24ceb9..1a45667402260 100644
> > --- a/kernel/rcu/tree_exp.h
> > +++ b/kernel/rcu/tree_exp.h
> > @@ -816,7 +816,7 @@ static int rcu_print_task_exp_stall(struct rcu_node *rnp)
> >    */
> >   void synchronize_rcu_expedited(void)
> >   {
> > -	bool boottime = (rcu_scheduler_active == RCU_SCHEDULER_INIT);
> > +	bool no_wq;
> >   	struct rcu_exp_work rew;
> >   	struct rcu_node *rnp;
> >   	unsigned long s;
> > @@ -841,9 +841,15 @@ void synchronize_rcu_expedited(void)
> >   	if (exp_funnel_lock(s))
> >   		return;  /* Someone else did our work for us. */
> > +	/* Don't use workqueue during boot or from an incoming CPU. */
> > +	preempt_disable();
> > +	no_wq = rcu_scheduler_active == RCU_SCHEDULER_INIT ||
> > +		!cpumask_test_cpu(smp_processor_id(), cpu_active_mask);
> > +	preempt_enable();
> > +
> >   	/* Ensure that load happens before action based on it. */
> > -	if (unlikely(boottime)) {
> > -		/* Direct call during scheduler init and early_initcalls(). */
> > +	if (unlikely(no_wq)) {
> > +		/* Direct call for scheduler init, early_initcall()s, and incoming CPUs. */
> >   		rcu_exp_sel_wait_wake(s);
> >   	} else {
> >   		/* Marshall arguments & schedule the expedited grace period. */
> > @@ -861,7 +867,7 @@ void synchronize_rcu_expedited(void)
> >   	/* Let the next expedited grace period start. */
> >   	mutex_unlock(&rcu_state.exp_mutex);
> > -	if (likely(!boottime))
> > +	if (likely(!no_wq))
> >   		destroy_work_on_stack(&rew.rew_work);
> >   }
> >   EXPORT_SYMBOL_GPL(synchronize_rcu_expedited);
> 
> Can we reach a condition after this change where no_wq = true and during
> rcu_stall report where exp_task = 0 list and exp_mask contain only this cpu
> ?

Hello, Mukesh, and thank you for looking this over!

At first glance, I do not believe that this can happen because the
expedited grace-period machinery avoids waiting on the current CPU.
(See sync_rcu_exp_select_node_cpus(), both the raw_smp_processor_id()
early in the function and the get_cpu() later in the function.)

But please let me know if I am missing something here.

But suppose that we could in fact reach this condition.  What bad thing
would happen?  Other than a resched_cpu() having been invoked several
times on a not-yet-online CPU, of course.  ;-)

							Thanx, Paul