[PATCH] rcu/sync: don't read rcu_sync->gp_count lockless

Oleg Nesterov
2024-05-12T11:26:21Z

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@kernel.org>
Signed-off-by: Oleg Nesterov <oleg@redhat.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