linux-kernel - [PATCH 25/48] rcu: Mark writes to rcu_sync ->gp

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Message-Id: <20240507093530.3043-26-urezki@gmail.com>
Date: Tue,  7 May 2024 11:35:07 +0200
From: "Uladzislau Rezki (Sony)" <urezki@...il.com>
To: "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>,
	Uladzislau Rezki <urezki@...il.com>,
	Oleksiy Avramchenko <oleksiy.avramchenko@...y.com>,
	Frederic Weisbecker <frederic@...nel.org>,
	Oleg Nesterov <oleg@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>
Subject: [PATCH 25/48] rcu: Mark writes to rcu_sync ->gp_count field

From: "Paul E. McKenney" <paulmck@...nel.org>

The rcu_sync structure's ->gp_count field is updated under the protection
of ->rss_lock, but read locklessly, and KCSAN noted the data race.
This commit therefore uses WRITE_ONCE() to do this update to clearly
document its racy nature.

Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
Cc: Oleg Nesterov <oleg@...hat.com>
Cc: Peter Zijlstra <peterz@...radead.org>
Signed-off-by: Uladzislau Rezki (Sony) <urezki@...il.com>
---
 kernel/rcu/sync.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/kernel/rcu/sync.c b/kernel/rcu/sync.c
index 86df878a2fee..6c2bd9001adc 100644
--- a/kernel/rcu/sync.c
+++ b/kernel/rcu/sync.c
@@ -122,7 +122,7 @@ void rcu_sync_enter(struct rcu_sync *rsp)
 		 * we are called at early boot time but this shouldn't happen.
 		 */
 	}
-	rsp->gp_count++;
+	WRITE_ONCE(rsp->gp_count, rsp->gp_count + 1);
 	spin_unlock_irq(&rsp->rss_lock);
 
 	if (gp_state == GP_IDLE) {
@@ -151,11 +151,15 @@ void rcu_sync_enter(struct rcu_sync *rsp)
  */
 void rcu_sync_exit(struct rcu_sync *rsp)
 {
+	int gpc;
+
 	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);
-	if (!--rsp->gp_count) {
+	gpc = rsp->gp_count - 1;
+	WRITE_ONCE(rsp->gp_count, gpc);
+	if (!gpc) {
 		if (rsp->gp_state == GP_PASSED) {
 			WRITE_ONCE(rsp->gp_state, GP_EXIT);
 			rcu_sync_call(rsp);
-- 
2.39.2