Re: linux-5.13.2: warning from kernel/rcu/tree_plugin.h:359

[Paul E. McKenney]

On Sun, Jul 18, 2021 at 11:51:36PM +0100, Matthew Wilcox wrote:
> On Sun, Jul 18, 2021 at 02:59:14PM -0700, Paul E. McKenney wrote:
> > > > https://lore.kernel.org/lkml/CAK2bqVK0Q9YcpakE7_Rc6nr-E4e2GnMOgi5jJj=_Eh_1k
> > > > EHLHA@xxxxxxxxxxxxxx/
> > 
> > But this one does show this warning in v5.12.17:
> > 
> > 	WARN_ON_ONCE(!preempt && rcu_preempt_depth() > 0);
> > 
> > This is in rcu_note_context_switch(), and could be caused by something
> > like a schedule() within an RCU read-side critical section.  This would
> > of course be RCU-usage bugs, given that you are not permitted to block
> > within an RCU read-side critical section.
> > 
> > I suggest checking the functions in the stack trace to see where the
> > rcu_read_lock() is hiding.  CONFIG_PROVE_LOCKING might also be helpful.
> 
> I'm not sure I see it in this stack trace.
> 
> Is it possible that there's something taking the rcu read lock in an
> interrupt handler, then returning from the interrupt handler without
> releasing the rcu lock?  Do we have debugging that would fire if
> somebody did this?

Lockdep should complain, but in the absence of lockdep I don't know
that anything would gripe in this situation.

Also, this is a preemptible kernel, so it is possible to trace
__rcu_read_lock(), if that helps.

							Thanx, Paul