linux-kernel - [PATCH rcu 1/8] rcu: Remove duplicate RCU exp QS report from rcu_report

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Message-Id: <20221019224659.2499511-1-paulmck@kernel.org>
Date:   Wed, 19 Oct 2022 15:46:52 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     rcu@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org, kernel-team@...com,
        rostedt@...dmis.org, Zqiang <qiang1.zhang@...el.com>,
        "Paul E . McKenney" <paulmck@...nel.org>
Subject: [PATCH rcu 1/8] rcu: Remove duplicate RCU exp QS report from rcu_report_dead()

From: Zqiang <qiang1.zhang@...el.com>

The rcu_report_dead() function invokes rcu_report_exp_rdp() in order
to force an immediate expedited quiescent state on the outgoing
CPU, and then it invokes rcu_preempt_deferred_qs() to provide any
required deferred quiescent state of either sort.  Because the call to
rcu_preempt_deferred_qs() provides the expedited RCU quiescent state if
requested, the call to rcu_report_exp_rdp() is potentially redundant.

One possible issue is a concurrent start of a new expedited RCU
grace period, but this situation is already handled correctly
by __sync_rcu_exp_select_node_cpus().  This function will detect
that the CPU is going offline via the error return from its call
to smp_call_function_single().  In that case, it will retry, and
eventually stop retrying due to rcu_report_exp_rdp() clearing the
->qsmaskinitnext bit corresponding to the target CPU.  As a result,
__sync_rcu_exp_select_node_cpus() will report the necessary quiescent
state after dealing with any remaining CPU.

This change assumes that control does not enter rcu_report_dead() within
an RCU read-side critical section, but then again, the surviving call
to rcu_preempt_deferred_qs() has always made this assumption.

This commit therefore removes the call to rcu_report_exp_rdp(), thus
relying on rcu_preempt_deferred_qs() to handle both normal and expedited
quiescent states.

Signed-off-by: Zqiang <qiang1.zhang@...el.com>
Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
---
 kernel/rcu/tree.c | 2 --
 1 file changed, 2 deletions(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 6bb8e72bc8151..0ca21ac0f0648 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. */
-- 
2.31.1.189.g2e36527f23