linux-kernel - [PATCH rcu 02/10] rcu: Remove READ_ONCE() for rdp->gpwrap access in __note_gp

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Message-ID: <20250305030505.94059-3-boqun.feng@gmail.com>
Date: Tue,  4 Mar 2025 19:04:57 -0800
From: Boqun Feng <boqun.feng@...il.com>
To: rcu@...r.kernel.org
Cc: Petr Mladek <pmladek@...e.com>,	Steven Rostedt <rostedt@...dmis.org>,
	John Ogness <john.ogness@...utronix.de>,
	Sergey Senozhatsky <senozhatsky@...omium.org>,
	"Paul E. McKenney" <paulmck@...nel.org>,
	Frederic Weisbecker <frederic@...nel.org>,
	Neeraj Upadhyay <neeraj.upadhyay@...nel.org>,
	Joel Fernandes <joel@...lfernandes.org>,
	Josh Triplett <josh@...htriplett.org>,	Boqun Feng <boqun.feng@...il.com>,
	Uladzislau Rezki <urezki@...il.com>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Lai Jiangshan <jiangshanlai@...il.com>,
	Zqiang <qiang.zhang1211@...il.com>,	Davidlohr Bueso <dave@...olabs.net>,
	Shuah Khan <shuah@...nel.org>,
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
	Clark Williams <clrkwllms@...nel.org>,
	Masami Hiramatsu <mhiramat@...nel.org>,	Yuntao Wang <ytcoode@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Rasmus Villemoes <linux@...musvillemoes.dk>,
	Guo Weikang <guoweikang.kernel@...il.com>,	KP Singh <kpsingh@...nel.org>,
	Huang Shijie <shijie@...amperecomputing.com>,
	Raul E Rangel <rrangel@...omium.org>,
	Joel Granados <joel.granados@...nel.org>,
	Anna Schumaker <anna.schumaker@...cle.com>,
	"Martin K. Petersen" <martin.petersen@...cle.com>,
	Thomas Weißschuh <linux@...ssschuh.net>,
	zhangguopeng <zhangguopeng@...inos.cn>,
	Thomas Gleixner <tglx@...utronix.de>,
	"Neeraj Upadhyay (AMD)" <neeraj.iitr10@...il.com>,
	linux-kernel@...r.kernel.org,	linux-kselftest@...r.kernel.org,
	linux-rt-devel@...ts.linux.dev,	Zilin Guan <zilinguan811@...il.com>
Subject: [PATCH rcu 02/10] rcu: Remove READ_ONCE() for rdp->gpwrap access in __note_gp_changes()

From: Zilin Guan <zilinguan811@...il.com>

There is one access to the per-CPU rdp->gpwrap field in the
__note_gp_changes() function that does not use READ_ONCE(), but all other
accesses do use READ_ONCE().  When using the 8*TREE03 and CONFIG_NR_CPUS=8
configuration, KCSAN found no data races at that point.  This is because
all calls to __note_gp_changes() hold rnp->lock, which excludes writes
to the rdp->gpwrap fields for all CPUs associated with that same leaf
rcu_node structure.

This commit therefore removes READ_ONCE() from rdp->gpwrap accesses
within the __note_gp_changes() function.

Signed-off-by: Zilin Guan <zilinguan811@...il.com>
Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
Signed-off-by: Boqun Feng <boqun.feng@...il.com>
---
 kernel/rcu/tree.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 475f31deed14..e4c0ce600b2b 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -1254,7 +1254,7 @@ static bool __note_gp_changes(struct rcu_node *rnp, struct rcu_data *rdp)
 
 	/* Handle the ends of any preceding grace periods first. */
 	if (rcu_seq_completed_gp(rdp->gp_seq, rnp->gp_seq) ||
-	    unlikely(READ_ONCE(rdp->gpwrap))) {
+	    unlikely(rdp->gpwrap)) {
 		if (!offloaded)
 			ret = rcu_advance_cbs(rnp, rdp); /* Advance CBs. */
 		rdp->core_needs_qs = false;
@@ -1268,7 +1268,7 @@ static bool __note_gp_changes(struct rcu_node *rnp, struct rcu_data *rdp)
 
 	/* Now handle the beginnings of any new-to-this-CPU grace periods. */
 	if (rcu_seq_new_gp(rdp->gp_seq, rnp->gp_seq) ||
-	    unlikely(READ_ONCE(rdp->gpwrap))) {
+	    unlikely(rdp->gpwrap)) {
 		/*
 		 * If the current grace period is waiting for this CPU,
 		 * set up to detect a quiescent state, otherwise don't
@@ -1283,7 +1283,7 @@ static bool __note_gp_changes(struct rcu_node *rnp, struct rcu_data *rdp)
 	rdp->gp_seq = rnp->gp_seq;  /* Remember new grace-period state. */
 	if (ULONG_CMP_LT(rdp->gp_seq_needed, rnp->gp_seq_needed) || rdp->gpwrap)
 		WRITE_ONCE(rdp->gp_seq_needed, rnp->gp_seq_needed);
-	if (IS_ENABLED(CONFIG_PROVE_RCU) && READ_ONCE(rdp->gpwrap))
+	if (IS_ENABLED(CONFIG_PROVE_RCU) && rdp->gpwrap)
 		WRITE_ONCE(rdp->last_sched_clock, jiffies);
 	WRITE_ONCE(rdp->gpwrap, false);
 	rcu_gpnum_ovf(rnp, rdp);
-- 
2.48.1