linux-kernel - [PATCH rcu 8/9] rcu: Eliminate lockless accesses to rcu_sync->gp

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Date: Tue,  4 Jun 2024 15:23:54 -0700
From: "Paul E. McKenney" <paulmck@...nel.org>
To: rcu@...r.kernel.org
Cc: linux-kernel@...r.kernel.org,
	kernel-team@...a.com,
	rostedt@...dmis.org,
	Oleg Nesterov <oleg@...hat.com>,
	"Paul E . McKenney" <paulmck@...nel.org>
Subject: [PATCH rcu 8/9] rcu: Eliminate lockless accesses to rcu_sync->gp_count

From: Oleg Nesterov <oleg@...hat.com>

The rcu_sync structure's ->gp_count field is always accessed under the
protection of that same structure's ->rss_lock field, with the exception
of a pair of WARN_ON_ONCE() calls just prior to acquiring that lock in
functions rcu_sync_exit() and rcu_sync_dtor().  These lockless accesses
are unnecessary and impair KCSAN's ability to catch bugs that might be
inserted via other lockless accesses.

This commit therefore moves those WARN_ON_ONCE() calls under the lock.

Signed-off-by: Oleg Nesterov <oleg@...hat.com>
Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
---
 kernel/rcu/sync.c | 12 ++++--------
 1 file changed, 4 insertions(+), 8 deletions(-)

diff --git a/kernel/rcu/sync.c b/kernel/rcu/sync.c
index 6c2bd9001adcd..da60a9947c005 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.
 		 */
 	}
-	WRITE_ONCE(rsp->gp_count, rsp->gp_count + 1);
+	rsp->gp_count++;
 	spin_unlock_irq(&rsp->rss_lock);
 
 	if (gp_state == GP_IDLE) {
@@ -151,15 +151,11 @@ 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);
-	gpc = rsp->gp_count - 1;
-	WRITE_ONCE(rsp->gp_count, gpc);
-	if (!gpc) {
+	WARN_ON_ONCE(rsp->gp_count == 0);
+	if (!--rsp->gp_count) {
 		if (rsp->gp_state == GP_PASSED) {
 			WRITE_ONCE(rsp->gp_state, GP_EXIT);
 			rcu_sync_call(rsp);
@@ -178,10 +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);
 	if (rsp->gp_state == GP_REPLAY)
 		WRITE_ONCE(rsp->gp_state, GP_EXIT);
 	gp_state = rsp->gp_state;
-- 
2.40.1