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
