rcu-tasks: Mark ->trc_reader_nesting data races

There are several ->trc_reader_nesting data races that are too
low-probability for KCSAN to notice, but which will happen sooner or
later.  This commit therefore marks these accesses, and comments one
that cannot race.
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

kernel/rcu/tasks.h

@@ -897,7 +897,7 @@ static void trc_read_check_handler(void *t_in)
 
 	// If the task is not in a read-side critical section, and
 	// if this is the last reader, awaken the grace-period kthread.
-	if (likely(!t->trc_reader_nesting)) {
+	if (likely(!READ_ONCE(t->trc_reader_nesting))) {
 		if (WARN_ON_ONCE(atomic_dec_and_test(&trc_n_readers_need_end)))
 			wake_up(&trc_wait);
 		// Mark as checked after decrement to avoid false
@@ -906,7 +906,7 @@ static void trc_read_check_handler(void *t_in)
 		goto reset_ipi;
 	}
 	// If we are racing with an rcu_read_unlock_trace(), try again later.
-	if (unlikely(t->trc_reader_nesting < 0)) {
+	if (unlikely(READ_ONCE(t->trc_reader_nesting) < 0)) {
 		if (WARN_ON_ONCE(atomic_dec_and_test(&trc_n_readers_need_end)))
 			wake_up(&trc_wait);
 		goto reset_ipi;
@@ -953,6 +953,7 @@ static bool trc_inspect_reader(struct task_struct *t, void *arg)
 			n_heavy_reader_ofl_updates++;
 		in_qs = true;
 	} else {
+		// The task is not running, so C-language access is safe.
 		in_qs = likely(!t->trc_reader_nesting);
 	}
 
@@ -985,7 +986,7 @@ static void trc_wait_for_one_reader(struct task_struct *t,
 	// The current task had better be in a quiescent state.
 	if (t == current) {
 		t->trc_reader_checked = true;
-		WARN_ON_ONCE(t->trc_reader_nesting);
+		WARN_ON_ONCE(READ_ONCE(t->trc_reader_nesting));
 		return;
 	}
 
@@ -1101,7 +1102,7 @@ static void show_stalled_task_trace(struct task_struct *t, bool *firstreport)
 		 ".I"[READ_ONCE(t->trc_ipi_to_cpu) > 0],
 		 ".i"[is_idle_task(t)],
 		 ".N"[cpu > 0 && tick_nohz_full_cpu(cpu)],
-		 t->trc_reader_nesting,
+		 READ_ONCE(t->trc_reader_nesting),
 		 " N"[!!t->trc_reader_special.b.need_qs],
 		 cpu);
 	sched_show_task(t);
@@ -1196,7 +1197,7 @@ static void rcu_tasks_trace_postgp(struct rcu_tasks *rtp)
 static void exit_tasks_rcu_finish_trace(struct task_struct *t)
 {
 	WRITE_ONCE(t->trc_reader_checked, true);
-	WARN_ON_ONCE(t->trc_reader_nesting);
+	WARN_ON_ONCE(READ_ONCE(t->trc_reader_nesting));
 	WRITE_ONCE(t->trc_reader_nesting, 0);
 	if (WARN_ON_ONCE(READ_ONCE(t->trc_reader_special.b.need_qs)))
 		rcu_read_unlock_trace_special(t, 0);