md/raid10: fix io loss while replacement replace rdev

hulk inclusion
category: bugfix
bugzilla: 188787, https://gitee.com/openeuler/kernel/issues/I78YIW
CVE: NA

--------------------------------

When we remove a disk which has replacement, first set rdev to NULL
and then set replacement to rdev, finally set replacement to NULL (see
raid10_remove_disk()). If io is submitted during the same time, it might
read both rdev and replacement as NULL, and io will not be submitted.

  rdev -> NULL
                        read rdev
  replacement -> NULL
                        read replacement

Fix it by reading replacement first and rdev later, meanwhile, use smp_mb()
to prevent memory reordering.

Fixes: 475b0321 ("md/raid10: writes should get directed to replacement as well as original.")
Signed-off-by: NLi Nan <linan122@huawei.com>
Reviewed-by: NYu Kuai <yukuai3@huawei.com>
Reviewed-by: NHou Tao <houtao1@huawei.com>

drivers/md/raid10.c

@@ -754,8 +754,16 @@ static struct md_rdev *read_balance(struct r10conf *conf,
 		rdev = rcu_dereference(conf->mirrors[disk].replacement);
 		if (rdev == NULL || test_bit(Faulty, &rdev->flags) ||
 		    test_bit(WantRemove, &rdev->flags) ||
-		    r10_bio->devs[slot].addr + sectors > rdev->recovery_offset)
+		    r10_bio->devs[slot].addr + sectors >
+		    rdev->recovery_offset) {
+			/*
+			 * Read replacement first to prevent reading both rdev
+			 * and replacement as NULL during replacement replace
+			 * rdev
+			 */
+			smp_mb();
 			rdev = rcu_dereference(conf->mirrors[disk].rdev);
+		}
 		if (rdev == NULL ||
 		    test_bit(WantRemove, &rdev->flags) ||
 		    test_bit(Faulty, &rdev->flags))
@@ -1363,9 +1371,15 @@ static void raid10_write_request(struct mddev *mddev, struct bio *bio,
 
 	for (i = 0;  i < conf->copies; i++) {
 		int d = r10_bio->devs[i].devnum;
-		struct md_rdev *rdev = rcu_dereference(conf->mirrors[d].rdev);
-		struct md_rdev *rrdev = rcu_dereference(
-			conf->mirrors[d].replacement);
+		struct md_rdev *rrdev, *rdev;
+
+		rrdev = rcu_dereference(conf->mirrors[d].replacement);
+		/*
+		 * Read replacement first to Prevent reading both rdev and
+		 * replacement as NULL during replacement replace rdev.
+		 */
+		smp_mb();
+		rdev = rcu_dereference(conf->mirrors[d].rdev);
 		if (rdev == rrdev)
 			rrdev = NULL;
 		if (rdev && unlikely(test_bit(Blocked, &rdev->flags))) {