srcu: Check for concurrent updates of heuristics

SRCU maintains the ->srcu_n_exp_nodelay and ->reschedule_count values
to guide heuristics governing auto-expediting of normal SRCU grace
periods and grace-period-state-machine delays.  This commit adds KCSAN
ASSERT_EXCLUSIVE_WRITER() calls to check for concurrent updates to
these fields.

Signed-off-by: "Paul E. McKenney" <[email protected]>
Signed-off-by: Neeraj Upadhyay <[email protected]>

Author: paulmckrcu

kernel/rcu/srcutree.c

@@ -629,6 +629,7 @@ static unsigned long srcu_get_delay(struct srcu_struct *ssp)
 		if (time_after(j, gpstart))
 			jbase += j - gpstart;
 		if (!jbase) {
+			ASSERT_EXCLUSIVE_WRITER(sup->srcu_n_exp_nodelay);
 			WRITE_ONCE(sup->srcu_n_exp_nodelay, READ_ONCE(sup->srcu_n_exp_nodelay) + 1);
 			if (READ_ONCE(sup->srcu_n_exp_nodelay) > srcu_max_nodelay_phase)
 				jbase = 1;
@@ -1819,6 +1820,7 @@ static void process_srcu(struct work_struct *work)
 	} else {
 		j = jiffies;
 		if (READ_ONCE(sup->reschedule_jiffies) == j) {
+			ASSERT_EXCLUSIVE_WRITER(sup->reschedule_count);
 			WRITE_ONCE(sup->reschedule_count, READ_ONCE(sup->reschedule_count) + 1);
 			if (READ_ONCE(sup->reschedule_count) > srcu_max_nodelay)
 				curdelay = 1;