ZFS: add mutex protection to metaslab_class_t.mc_historgram

The mc_histogram fields were unprotected when that code was first
written in "Illumos 4976-4984 - metaslab improvements" (OpenZFS
f3a7f66).  The lock wasn't added until
3dfb57a, though it's unclear exactly
which fields it's supposed to protect.  In any case, it wasn't until
vdev_load was parallelized that any code attempted concurrent access to
those fields.

module/zfs/metaslab.c

@@ -522,6 +522,7 @@ metaslab_class_histogram_verify(metaslab_class_t *mc)
 	mc_hist = kmem_zalloc(sizeof (uint64_t) * RANGE_TREE_HISTOGRAM_SIZE,
 	    KM_SLEEP);
 
+	mutex_enter(&mc->mc_lock);
 	for (int c = 0; c < rvd->vdev_children; c++) {
 		vdev_t *tvd = rvd->vdev_child[c];
 		metaslab_group_t *mg = tvd->vdev_mg;
@@ -542,6 +543,7 @@ metaslab_class_histogram_verify(metaslab_class_t *mc)
 	for (i = 0; i < RANGE_TREE_HISTOGRAM_SIZE; i++)
 		VERIFY3U(mc_hist[i], ==, mc->mc_histogram[i]);
 
+	mutex_exit(&mc->mc_lock);
 	kmem_free(mc_hist, sizeof (uint64_t) * RANGE_TREE_HISTOGRAM_SIZE);
 }
 
@@ -1053,12 +1055,14 @@ metaslab_group_histogram_add(metaslab_group_t *mg, metaslab_t *msp)
 		return;
 
 	mutex_enter(&mg->mg_lock);
+	mutex_enter(&mc->mc_lock);
 	for (int i = 0; i < SPACE_MAP_HISTOGRAM_SIZE; i++) {
 		mg->mg_histogram[i + ashift] +=
 		    msp->ms_sm->sm_phys->smp_histogram[i];
 		mc->mc_histogram[i + ashift] +=
 		    msp->ms_sm->sm_phys->smp_histogram[i];
 	}
+	mutex_exit(&mc->mc_lock);
 	mutex_exit(&mg->mg_lock);
 }
 
@@ -1073,6 +1077,7 @@ metaslab_group_histogram_remove(metaslab_group_t *mg, metaslab_t *msp)
 		return;
 
 	mutex_enter(&mg->mg_lock);
+	mutex_enter(&mc->mc_lock);
 	for (int i = 0; i < SPACE_MAP_HISTOGRAM_SIZE; i++) {
 		ASSERT3U(mg->mg_histogram[i + ashift], >=,
 		    msp->ms_sm->sm_phys->smp_histogram[i]);
@@ -1084,6 +1089,7 @@ metaslab_group_histogram_remove(metaslab_group_t *mg, metaslab_t *msp)
 		mc->mc_histogram[i + ashift] -=
 		    msp->ms_sm->sm_phys->smp_histogram[i];
 	}
+	mutex_exit(&mc->mc_lock);
 	mutex_exit(&mg->mg_lock);
 }