linux-kernel - [PATCH] rcu/sync: don't read rcu_sync->gp

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Message-ID: <20240512111948.GC7541@redhat.com>
Date: Sun, 12 May 2024 13:19:48 +0200
From: Oleg Nesterov <oleg@...hat.com>
To: "Uladzislau Rezki (Sony)" <urezki@...il.com>,
	"Paul E . McKenney" <paulmck@...nel.org>
Cc: RCU <rcu@...r.kernel.org>, Neeraj upadhyay <Neeraj.Upadhyay@....com>,
	Boqun Feng <boqun.feng@...il.com>, Hillf Danton <hdanton@...a.com>,
	Joel Fernandes <joel@...lfernandes.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Oleksiy Avramchenko <oleksiy.avramchenko@...y.com>,
	Frederic Weisbecker <frederic@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>
Subject: [PATCH] rcu/sync: don't read rcu_sync->gp_count lockless

rcu_sync->gp_count is updated under the protection of ->rss_lock but read
locklessly by the WARN_ON() checks, and KCSAN noted the data race.

Move these WARN_ON_ONCE()'s under the lock and remove the no longer needed
READ_ONCE().

Reported-by: "Paul E. McKenney" <paulmck@...nel.org>
Signed-off-by: Oleg Nesterov <oleg@...hat.com>
---
 kernel/rcu/sync.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/kernel/rcu/sync.c b/kernel/rcu/sync.c
index 86df878a2fee..b50fde082198 100644
--- a/kernel/rcu/sync.c
+++ b/kernel/rcu/sync.c
@@ -152,9 +152,9 @@ void rcu_sync_enter(struct rcu_sync *rsp)
 void rcu_sync_exit(struct rcu_sync *rsp)
 {
 	WARN_ON_ONCE(READ_ONCE(rsp->gp_state) == GP_IDLE);
-	WARN_ON_ONCE(READ_ONCE(rsp->gp_count) == 0);
 
 	spin_lock_irq(&rsp->rss_lock);
+	WARN_ON_ONCE(rsp->gp_count == 0);
 	if (!--rsp->gp_count) {
 		if (rsp->gp_state == GP_PASSED) {
 			WRITE_ONCE(rsp->gp_state, GP_EXIT);
@@ -174,10 +174,10 @@ void rcu_sync_dtor(struct rcu_sync *rsp)
 {
 	int gp_state;
 
-	WARN_ON_ONCE(READ_ONCE(rsp->gp_count));
 	WARN_ON_ONCE(READ_ONCE(rsp->gp_state) == GP_PASSED);
 
 	spin_lock_irq(&rsp->rss_lock);
+	WARN_ON_ONCE(rsp->gp_count != 0);
 	if (rsp->gp_state == GP_REPLAY)
 		WRITE_ONCE(rsp->gp_state, GP_EXIT);
 	gp_state = rsp->gp_state;
-- 
2.25.1.362.g51ebf55