md/raid10: add rcu protection in raid10_status.

mirrors[].rdev can become NULL at any point unless:
 - a counted reference is held
 - ->reconfig_mutex is held, or
 - rcu_read_lock() is held

raid10_status holds none of these.  So add rcu_read_lock()
protection.

Signed-off-by: NeilBrown <neilb@suse.com>
Signed-off-by: Shaohua Li <shli@fb.com>

diff --git a/drivers/md/raid10.c b/drivers/md/raid10.c
index 6044864..334a701 100644 (file)
--- a/drivers/md/raid10.c
+++ b/drivers/md/raid10.c
@@ -1495,10 +1495,12 @@ static void raid10_status(struct seq_file *seq, struct mddev *mddev)
        }
        seq_printf(seq, " [%d/%d] [", conf->geo.raid_disks,
                                        conf->geo.raid_disks - mddev->degraded);
-       for (i = 0; i < conf->geo.raid_disks; i++)
-               seq_printf(seq, "%s",
-                             conf->mirrors[i].rdev &&
-                             test_bit(In_sync, &conf->mirrors[i].rdev->flags) ? "U" : "_");
+       rcu_read_lock();
+       for (i = 0; i < conf->geo.raid_disks; i++) {
+               struct md_rdev *rdev = rcu_dereference(conf->mirrors[i].rdev);
+               seq_printf(seq, "%s", rdev && test_bit(In_sync, &rdev->flags) ? "U" : "_");
+       }
+       rcu_read_unlock();
        seq_printf(seq, "]");
 }