[PATCH 25/48] rcu: Mark writes to rcu_sync ->gp_count field

Uladzislau Rezki (Sony)
2024-05-07T09:35:58Z

From: "Paul E. McKenney" <paulmck@kernel.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@kernel.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.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