Re: Can RCU stall lead to hard lockups?

["Serge E. Hallyn" <serge@hallyn.com>]

Quoting Paul E. McKenney (paulmck@linux.vnet.ibm.com):
> On Tue, Jan 09, 2018 at 06:11:14AM -0800, Tejun Heo wrote:
> > Hello, Paul.
> > 
> > On Mon, Jan 08, 2018 at 08:24:25PM -0800, Paul E. McKenney wrote:
> > > > I don't know the RCU code at all but it *looks* like the first CPU is
> > > > taking a sweet while flushing printk buffer while holding a lock (the
> > > > console is IPMI serial console, which faithfully emulates 115200 baud
> > > > rate), and everyone else seems stuck waiting for that spinlock in
> > > > rcu_check_callbacks().
> > > > 
> > > > Does this sound possible?
> > > 
> > > 115200 baud?  Ouch!!!  That -will- result in trouble from console
> > > printing, and often also in RCU CPU stall warnings.
> > 
> > It could even be slower than 115200, and we occassionally see RCU
> > stall warnings caused by printk storms, for example, while the kernel
> > is trying to dump a lot of info after an OOM.  That's an issue we
> > probably want to improve from printk side; however, they don't usually
> > lead to NMI hard lockup detector kicking in and crashing the machine,
> > which is the peculiarity here.
> > 
> > Hmmm... show_state_filter(), the function which dumps all task
> > backtraces, share a similar problem and it avoids it by explicitly
> > calling touch_nmi_watchdog().  Maybe we can do something like the
> > following from RCU too?
> 
> If this fixes things for you, I would welcome such a patch.

Hi - would this also be relevant to 4.9-stable and 4.4-stable, or
has something elsewhere changed after 4.9 that actually triggers this?

thanks,
-serge

> 							Thanx, Paul
> 
> > diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
> > index db85ca3..3c4c4d3 100644
> > --- a/kernel/rcu/tree_plugin.h
> > +++ b/kernel/rcu/tree_plugin.h
> > @@ -561,8 +561,14 @@ static void rcu_print_detail_task_stall_rnp(struct rcu_node *rnp)
> >  	}
> >  	t = list_entry(rnp->gp_tasks->prev,
> >  		       struct task_struct, rcu_node_entry);
> > -	list_for_each_entry_continue(t, &rnp->blkd_tasks, rcu_node_entry)
> > +	list_for_each_entry_continue(t, &rnp->blkd_tasks, rcu_node_entry) {
> > +		touch_nmi_watchdog();
> > +		/*
> > +		 * We could be printing a lot of these messages while
> > +		 * holding a spinlock.  Avoid triggering hard lockup.
> > +		 */
> >  		sched_show_task(t);
> > +	}
> >  	raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
> >  }
> > 
> > @@ -1678,6 +1684,12 @@ static void print_cpu_stall_info(struct rcu_state *rsp, int cpu)
> >  	char *ticks_title;
> >  	unsigned long ticks_value;
> > 
> > +	/*
> > +	 * We could be printing a lot of these messages while holding a
> > +	 * spinlock.  Avoid triggering hard lockup.
> > +	 */
> > +	touch_nmi_watchdog();
> > +
> >  	if (rsp->gpnum == rdp->gpnum) {
> >  		ticks_title = "ticks this GP";
> >  		ticks_value = rdp->ticks_this_gp;
> >