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SubjectRe: [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@intel.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
>

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