RE: [PATCH] rcu: Remove duplicate RCU exp QS report from rcu_report_dead()

[Zhang, Qiang1]

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.

In the following code, the current CPU will report QS if (rnp->qsmask & mask) return true.
it means that the WARN_ON_ONCE(!rcu_rdp_cpu_online(rdp)) is not trigger.

if (rnp->qsmask & mask) { /* RCU waiting on outgoing CPU? */
                /* Report quiescent state -before- changing ->qsmaskinitnext! */
                rcu_disable_urgency_upon_qs(rdp);
                rcu_report_qs_rnp(mask, rnp, rnp->gp_seq, flags);
                raw_spin_lock_irqsave_rcu_node(rnp, flags);
        }
 WRITE_ONCE(rnp->qsmaskinitnext, rnp->qsmaskinitnext & ~mask);

>
>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()?

1.When a new expedited grace period that starts just as this CPU call rcu_report_dead(),
if in this time, this CPU rcu_data structure's cpu_no_qs.b.exp is not set true, 
the rcu_preempt_deferred_qs() will not call rcu_report_exp_rdp().  but when call
rcu_report_dead(),  this CPU have been offline(cpu_is_offline(this CPU) return true).

2.In __sync_rcu_exp_select_node_cpus(), invoke smp_call_function_single() for this
CPU will return -ENXIO,  and then check  (rnp->qsmaskinitnext & mask) and (rnp->expmask & mask) 

3.If in this time, the rcu_report_dead() has not yet clear CPU mask from rnp->qsmaskinitnext,
we will redo 2 step,  recall smp_call_function_single(), but is always return -ENXIO, 
and recheck (rnp->qsmaskinitnext & mask) and (rnp->expmask & mask), until rcu_report_dead()
clear CPU mask from rnp->qsmaskinitnext.  

Therefore, the __sync_rcu_exp_select_node_cpus() will call rcu_report_exp_cpu_mult()
to report this offline CPU's exp QS 


Thanks
Zqiang

>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
>