linux-kernel - Re: [PATCH] rcu: Remove duplicate RCU exp QS report from rcu_report

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Date:   Tue, 30 Aug 2022 02:06:23 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Zqiang <qiang1.zhang@...el.com>
Cc:     frederic@...nel.org, quic_neeraju@...cinc.com,
        joel@...lfernandes.org, rcu@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] rcu: Remove duplicate RCU exp QS report from
 rcu_report_dead()

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@...el.com>

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
>