On Tue, Oct 13, 2020 at 12:44:50PM +0200, Peter Zijlstra wrote: > On Tue, Oct 13, 2020 at 12:34:06PM +0200, Peter Zijlstra wrote: > > On Mon, Oct 12, 2020 at 02:28:12PM -0700, Paul E. McKenney wrote: > > > It is certainly an accident waiting to happen. Would something like > > > the following make sense? > > > > Sadly no. Hey, I was hoping! ;-) > > > ------------------------------------------------------------------------ > > > > > > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c > > > index bfd38f2..52a63bc 100644 > > > --- a/kernel/rcu/tree.c > > > +++ b/kernel/rcu/tree.c > > > @@ -4067,6 +4067,7 @@ void rcu_cpu_starting(unsigned int cpu) > > > > > > rnp = rdp->mynode; > > > mask = rdp->grpmask; > > > + lockdep_off(); > > > raw_spin_lock_irqsave_rcu_node(rnp, flags); > > > WRITE_ONCE(rnp->qsmaskinitnext, rnp->qsmaskinitnext | mask); > > > newcpu = !(rnp->expmaskinitnext & mask); > > > @@ -4086,6 +4087,7 @@ void rcu_cpu_starting(unsigned int cpu) > > > } else { > > > raw_spin_unlock_irqrestore_rcu_node(rnp, flags); > > > } > > > + lockdep_on(); > > > smp_mb(); /* Ensure RCU read-side usage follows above initialization. */ > > > } > > > > This will just shut it up, but will not fix the actual problem of that > > spin-lock ending up in trace_lock_acquire() which relies on RCU which > > isn't looking. > > > > What we need here is to supress tracing not lockdep. Let me consider. OK, I certainly didn't think in those terms. > We appear to have a similar problem with rcu_report_dead(), it's > raw_spin_unlock()s can end up in trace_lock_release() while we just > killed RCU. In theory, rcu_report_dead() is just fine. The reason is that a new grace period that is ignoring the outgoing CPU cannot start until after: 1. This CPU releases the leaf rcu_node ->lock -and- 2. The grace-period kthread acquires this same lock. Multiple times. In practice, too bad about those diagnostics! :-( So good catch!!! Thanx, Paul
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