linux-kernel - [PATCH v2 10/13] rcu: Permit start_poll_synchronize_rcu

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Message-Id: <20230325173316.3118674-11-joel@joelfernandes.org>
Date:   Sat, 25 Mar 2023 17:33:13 +0000
From:   "Joel Fernandes (Google)" <joel@...lfernandes.org>
To:     linux-kernel@...r.kernel.org,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Frederic Weisbecker <frederic@...nel.org>,
        Neeraj Upadhyay <quic_neeraju@...cinc.com>,
        Joel Fernandes <joel@...lfernandes.org>,
        Josh Triplett <josh@...htriplett.org>,
        Boqun Feng <boqun.feng@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Zqiang <qiang1.zhang@...el.com>
Cc:     rcu@...r.kernel.org
Subject: [PATCH v2 10/13] rcu: Permit start_poll_synchronize_rcu_expedited() to be invoked early

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

According to the commit log of the patch that added it to the kernel,
start_poll_synchronize_rcu_expedited() can be invoked very early, as
in long before rcu_init() has been invoked.  But before rcu_init(),
the rcu_data structure's ->mynode field has not yet been initialized.
This means that the start_poll_synchronize_rcu_expedited() function's
attempt to set the CPU's leaf rcu_node structure's ->exp_seq_poll_rq
field will result in a segmentation fault.

This commit therefore causes start_poll_synchronize_rcu_expedited() to
set ->exp_seq_poll_rq only after rcu_init() has initialized all CPUs'
rcu_data structures' ->mynode fields.  It also removes the check from
the rcu_init() function so that start_poll_synchronize_rcu_expedited(
is unconditionally invoked.  Yes, this might result in an unnecessary
boot-time grace period, but this is down in the noise.

Signed-off-by: Zqiang <qiang1.zhang@...el.com>
Reviewed-by: Frederic Weisbecker <frederic@...nel.org>
Reviewed-by: Joel Fernandes (Google) <joel@...lfernandes.org>
Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
Signed-off-by: Joel Fernandes (Google) <joel@...lfernandes.org>
---
 kernel/rcu/tree.c     | 5 ++---
 kernel/rcu/tree_exp.h | 5 +++--
 2 files changed, 5 insertions(+), 5 deletions(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index e80e8f128c57..90d54571126a 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -4942,9 +4942,8 @@ void __init rcu_init(void)
 	else
 		qovld_calc = qovld;
 
-	// Kick-start any polled grace periods that started early.
-	if (!(per_cpu_ptr(&rcu_data, cpu)->mynode->exp_seq_poll_rq & 0x1))
-		(void)start_poll_synchronize_rcu_expedited();
+	// Kick-start in case any polled grace periods started early.
+	(void)start_poll_synchronize_rcu_expedited();
 
 	rcu_test_sync_prims();
 }
diff --git a/kernel/rcu/tree_exp.h b/kernel/rcu/tree_exp.h
index 7cc4856da081..5343f32e7d67 100644
--- a/kernel/rcu/tree_exp.h
+++ b/kernel/rcu/tree_exp.h
@@ -1066,9 +1066,10 @@ unsigned long start_poll_synchronize_rcu_expedited(void)
 	if (rcu_init_invoked())
 		raw_spin_lock_irqsave(&rnp->exp_poll_lock, flags);
 	if (!poll_state_synchronize_rcu(s)) {
-		rnp->exp_seq_poll_rq = s;
-		if (rcu_init_invoked())
+		if (rcu_init_invoked()) {
+			rnp->exp_seq_poll_rq = s;
 			queue_work(rcu_gp_wq, &rnp->exp_poll_wq);
+		}
 	}
 	if (rcu_init_invoked())
 		raw_spin_unlock_irqrestore(&rnp->exp_poll_lock, flags);
-- 
2.40.0.348.gf938b09366-goog