rcu: Add WRITE_ONCE() to rcu_state ->gp_start

The rcu_state structure's ->gp_start field is read locklessly, so this
commit adds the WRITE_ONCE() to an update in order to provide proper
documentation and READ_ONCE()/WRITE_ONCE() pairing.

This data race was reported by KCSAN.  Not appropriate for backporting
due to failure being unlikely.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

diff --git a/kernel/rcu/tree_stall.h b/kernel/rcu/tree_stall.h
index bca637b..488b71d 100644 (file)
--- a/kernel/rcu/tree_stall.h
+++ b/kernel/rcu/tree_stall.h
@@ -102,7 +102,7 @@ static void record_gp_stall_check_time(void)
        unsigned long j = jiffies;
        unsigned long j1;
 
-       rcu_state.gp_start = j;
+       WRITE_ONCE(rcu_state.gp_start, j);
        j1 = rcu_jiffies_till_stall_check();
        /* Record ->gp_start before ->jiffies_stall. */
        smp_store_release(&rcu_state.jiffies_stall, j + j1); /* ^^^ */