On Tue, Oct 13, 2020 at 12:27:15PM +0200, Peter Zijlstra wrote: > On Mon, Oct 12, 2020 at 11:11:10AM +0800, Boqun Feng wrote: > > > I think this happened because in this commit debug_lockdep_rcu_enabled() > > didn't adopt to the change that made lockdep_recursion a percpu > > variable? > > > > Qian, mind to try the following? > > > > Although, arguably the problem still exists, i.e. we still have an RCU > > read-side critical section inside lock_acquire(), which may be called on > > There is actual RCU usage from the trace_lock_acquire(). > > > a yet-to-online CPU, which RCU doesn't watch. I think this used to be OK > > because we don't "free" anything from lockdep, IOW, there is no > > synchronize_rcu() or call_rcu() that _needs_ to wait for the RCU > > read-side critical sections inside lockdep. But now we lock class > > recycling, so it might be a problem. > > > > That said, currently validate_chain() and lock class recycling are > > mutually excluded via graph_lock, so we are safe for this one ;-) > > We should have a comment on that somewhere, could you write one? > Sure, I will write something tomorrow. Regards, Boqun > > ----------->8 > > diff --git a/kernel/rcu/update.c b/kernel/rcu/update.c > > index 39334d2d2b37..35d9bab65b75 100644 > > --- a/kernel/rcu/update.c > > +++ b/kernel/rcu/update.c > > @@ -275,8 +275,8 @@ EXPORT_SYMBOL_GPL(rcu_callback_map); > > > > noinstr int notrace debug_lockdep_rcu_enabled(void) > > { > > - return rcu_scheduler_active != RCU_SCHEDULER_INACTIVE && debug_locks && > > - current->lockdep_recursion == 0; > > + return rcu_scheduler_active != RCU_SCHEDULER_INACTIVE && > > + __lockdep_enabled; > > } > > EXPORT_SYMBOL_GPL(debug_lockdep_rcu_enabled); > > Urgh, I didn't expect (and forgot to grep) lockdep_recursion users > outside of lockdep itself :/ It looks like this is indeed the only one. > >