On Tue, Aug 30, 2022 at 04:31:51PM +0800, Zqiang wrote: > For PREEMPT_RCU, the rcu_report_dead() is invoked means that the > outgoing CPU mask is clear from leaf rcu_node and has no further > need of RCU, so invoke rcu_preempt_depth() return value is always > zero in rcu_report_dead(), if the current outgoing CPU rcu_data > structure's cpu_no_qs.b.exp is true, the rcu_preempt_deferred_qs() > will invoke rcu_report_exp_rdp() to report exp QS. > > for non-PREEMPT_RCU, the rcu_preempt_deferred_qs() is equivalent to > rcu_report_exp_rdp(). > > Signed-off-by: Zqiang <qiang1.zhang@xxxxxxxxx> Nice! One question... Currently, for PREEMPT_RCU, the outgoing CPU silently reports a quiescent state even if there was a bug that resulted in that CPU still being in an RCU read-side critical section. With your change, the outgoing CPU would silently refuse to report a quiescent state. Is there something along the CPU-offline code path that already complains about this situation? If not, I believe that the first WARN_ON_ONCE() in rcu_implicit_dynticks_qs() would complain. Could you please try this, just so we know what happens in this case? One way of forcing this would be to do rcu_read_lock() just before the call to rcu_report_dead(), though other diagnostics might require that rcu_read_lock() to be earlier in the code. Another question in both cases... There is a more subtle change where the old code ignores rdp->cpu_no_qs.b.exp (thus invoking rcu_report_exp_rdp() unconditionally) and the new code avoids invoking rcu_report_exp_rdp() unless this is set. How does this interact with a new expedited grace period that starts just as this CPU calls rcu_report_dead()? The expedited grace-period code in __sync_rcu_exp_select_node_cpus() is of special concern here. Thanx, Paul > --- > kernel/rcu/tree.c | 2 -- > 1 file changed, 2 deletions(-) > > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c > index 6bb8e72bc815..0ca21ac0f064 100644 > --- a/kernel/rcu/tree.c > +++ b/kernel/rcu/tree.c > @@ -4276,8 +4276,6 @@ void rcu_report_dead(unsigned int cpu) > // Do any dangling deferred wakeups. > do_nocb_deferred_wakeup(rdp); > > - /* QS for any half-done expedited grace period. */ > - rcu_report_exp_rdp(rdp); > rcu_preempt_deferred_qs(current); > > /* Remove outgoing CPU from mask in the leaf rcu_node structure. */ > -- > 2.25.1 >
