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>

diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h
index 3a8fec505a2e8d04467b46edd29d43bfbe7681b4..2d3dc340b25af0e64f7d306b893e8485ca057acb 100644 (file)
--- a/kernel/rcu/tasks.h
+++ b/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);
        }
 
@@ -987,7 +988,7 @@ static void trc_wait_for_one_reader(struct task_struct *t,
        if (t == current) {
                t->trc_reader_checked = true;
                trc_del_holdout(t);
-               WARN_ON_ONCE(t->trc_reader_nesting);
+               WARN_ON_ONCE(READ_ONCE(t->trc_reader_nesting));
                return;
        }
 
@@ -1103,7 +1104,7 @@ 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);
@@ -1198,7 +1199,7 @@ 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);