Re: [PATCH] tracing: Add trace_trigger kernel command line option

[Paul E. McKenney]

On Thu, Oct 20, 2022 at 01:05:28PM -0400, Steven Rostedt wrote:
> On Thu, 20 Oct 2022 12:33:57 -0400
> Steven Rostedt <rostedt@xxxxxxxxxxx> wrote:
> 
> > diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
> > index c03fd7037add..79ac31a6a87b 100644
> > --- a/kernel/trace/trace.c
> > +++ b/kernel/trace/trace.c
> > @@ -2748,9 +2748,12 @@ void trace_buffered_event_disable(void)
> >  			       disable_trace_buffered_event, NULL, 1);
> >  	preempt_enable();
> >  
> > +	if (!irqs_disabled())
> > +		printk("IRQS DISABLED!! before %s:%d\n", __func__, __LINE__);
> >  	/* Wait for all current users to finish */
> > -	if (!early_boot_irqs_disabled)
> > -		synchronize_rcu();
> > +	synchronize_rcu();
> > +	if (!irqs_disabled())
> > +		printk("IRQS DISABLED!! after %s:%d\n", __func__, __LINE__);
> >  
> >  	for_each_tracing_cpu(cpu) {
> >  		free_page((unsigned long)per_cpu(trace_buffered_event, cpu));
> 
> 
> Continuing the above printk()s I found the culprit.
> 
> synchronize_rcu() {
>     rcu_poll_gp_seq_start_unlocked() {
> 	struct rcu_node *rnp = rcu_get_root();
> 
> 	if (rcu_init_invoked()) {
> 		lockdep_assert_irqs_enabled();
> 		raw_spin_lock_irq_rcu_node(rnp);
> 	}
> 	rcu_poll_gp_seq_start(snap);
> 	if (rcu_init_invoked())
> 		raw_spin_unlock_irq_rcu_node(rnp);  <<-- Interrupts enabled here
>    }

Thank you for digging into this!

Does the following help?

							Thanx, Paul

------------------------------------------------------------------------

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index abc615808b6e8..45ad9ef00d647 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -1425,30 +1425,32 @@ static void rcu_poll_gp_seq_end(unsigned long *snap)
 // where caller does not hold the root rcu_node structure's lock.
 static void rcu_poll_gp_seq_start_unlocked(unsigned long *snap)
 {
+	unsigned long flags;
 	struct rcu_node *rnp = rcu_get_root();
 
 	if (rcu_init_invoked()) {
 		lockdep_assert_irqs_enabled();
-		raw_spin_lock_irq_rcu_node(rnp);
+		raw_spin_lock_irqsave_rcu_node(rnp, flags);
 	}
 	rcu_poll_gp_seq_start(snap);
 	if (rcu_init_invoked())
-		raw_spin_unlock_irq_rcu_node(rnp);
+		raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
 }
 
 // Make the polled API aware of the end of a grace period, but where
 // caller does not hold the root rcu_node structure's lock.
 static void rcu_poll_gp_seq_end_unlocked(unsigned long *snap)
 {
+	unsigned long flags;
 	struct rcu_node *rnp = rcu_get_root();
 
 	if (rcu_init_invoked()) {
 		lockdep_assert_irqs_enabled();
-		raw_spin_lock_irq_rcu_node(rnp);
+		raw_spin_lock_irqsave_rcu_node(rnp, flags);
 	}
 	rcu_poll_gp_seq_end(snap);
 	if (rcu_init_invoked())
-		raw_spin_unlock_irq_rcu_node(rnp);
+		raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
 }
 
 /*