rcu: Mark writes to rcu_sync ->gp_count field

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>

diff --git a/kernel/rcu/sync.c b/kernel/rcu/sync.c
index 86df878a2fee8b0b78718d8158c23bbb8137fb6f..6c2bd9001adcdbede753450918e416b9bd4db5e1 100644 (file)
--- 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);