zfs: read all disk labels concurrently in vdev_label_read_config

This is similar to what we already do in vdev_geom_read_config.

openzfs/zfs#11470

Sponsored by:   Axcient

sys/cddl/contrib/opensolaris/uts/common/fs/zfs/vdev_label.c

@@ -577,9 +577,9 @@ vdev_label_read_config(vdev_t *vd, uint64_t txg)
 {
 	spa_t *spa = vd->vdev_spa;
 	nvlist_t *config = NULL;
-	vdev_phys_t *vp;
-	abd_t *vp_abd;
-	zio_t *zio;
+	vdev_phys_t *vp[VDEV_LABELS];
+	abd_t *vp_abd[VDEV_LABELS];
+	zio_t *zio[VDEV_LABELS];
 	uint64_t best_txg = 0;
 	uint64_t label_txg = 0;
 	int error = 0;
@@ -591,21 +591,24 @@ vdev_label_read_config(vdev_t *vd, uint64_t txg)
 	if (!vdev_readable(vd))
 		return (NULL);
 
-	vp_abd = abd_alloc_linear(sizeof (vdev_phys_t), B_TRUE);
-	vp = abd_to_buf(vp_abd);
+	for (int l = 0; l < VDEV_LABELS; l++) {
+		vp_abd[l] = abd_alloc_linear(sizeof (vdev_phys_t), B_TRUE);
+		vp[l] = abd_to_buf(vp_abd[l]);
+	}
 
 retry:
 	for (int l = 0; l < VDEV_LABELS; l++) {
-		nvlist_t *label = NULL;
-
-		zio = zio_root(spa, NULL, NULL, flags);
+		zio[l] = zio_root(spa, NULL, NULL, flags);
 
-		vdev_label_read(zio, vd, l, vp_abd,
-		    offsetof(vdev_label_t, vl_vdev_phys),
-		    sizeof (vdev_phys_t), NULL, NULL, flags);
+		vdev_label_read(zio[l], vd, l, vp_abd[l],
+		    offsetof(vdev_label_t, vl_vdev_phys), sizeof (vdev_phys_t),
+		    NULL, NULL, flags);
+	}
+	for (int l = 0; l < VDEV_LABELS; l++) {
+		nvlist_t *label = NULL;
 
-		if (zio_wait(zio) == 0 &&
-		    nvlist_unpack(vp->vp_nvlist, sizeof (vp->vp_nvlist),
+		if (zio_wait(zio[l]) == 0 &&
+		    nvlist_unpack(vp[l]->vp_nvlist, sizeof (vp[l]->vp_nvlist),
 		    &label, 0) == 0) {
 			/*
 			 * Auxiliary vdevs won't have txg values in their
@@ -618,6 +621,8 @@ vdev_label_read_config(vdev_t *vd, uint64_t txg)
 			    ZPOOL_CONFIG_POOL_TXG, &label_txg);
 			if ((error || label_txg == 0) && !config) {
 				config = label;
+				for (l++; l < VDEV_LABELS; l++)
+					zio_wait(zio[l]);
 				break;
 			} else if (label_txg <= txg && label_txg > best_txg) {
 				best_txg = label_txg;
@@ -646,7 +651,9 @@ vdev_label_read_config(vdev_t *vd, uint64_t txg)
 		    (u_longlong_t)txg);
 	}
 
-	abd_free(vp_abd);
+	for (int l = 0; l < VDEV_LABELS; l++) {
+		abd_free(vp_abd[l]);
+	}
 
 	return (config);
 }