Fix ztest deadlock in spa_vdev_remove()

This patch corrects an issue where spa_vdev_remove() would
call spa_history_log_internal() while holding the spa config
lock. This function may decide to block until the next txg if
the current one seems too full. However, since the thread is
holding the config log, the txg sync thread cannot progress
and the system ends up deadlocked. This patch simply moves
all calls to spa_history_log_internal() outside of the config
lock.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Tom Caputi <tcaputi@datto.com>
Closes #8162

module/zfs/vdev_removal.c

@@ -1894,10 +1894,6 @@ spa_vdev_remove_log(vdev_t *vd, uint64_t *txg)
 	vdev_dirty_leaves(vd, VDD_DTL, *txg);
 	vdev_config_dirty(vd);
 
-	spa_history_log_internal(spa, "vdev remove", NULL,
-	    "%s vdev %llu (log) %s", spa_name(spa), vd->vdev_id,
-	    (vd->vdev_path != NULL) ? vd->vdev_path : "-");
-
 	spa_vdev_config_exit(spa, NULL, *txg, 0, FTAG);
 
 	*txg = spa_vdev_config_enter(spa);
@@ -2122,6 +2118,7 @@ spa_vdev_remove(spa_t *spa, uint64_t guid, boolean_t unspare)
 	int error = 0;
 	boolean_t locked = MUTEX_HELD(&spa_namespace_lock);
 	sysevent_t *ev = NULL;
+	char *vd_type = NULL, *vd_path = NULL;
 
 	ASSERT(spa_writeable(spa));
 
@@ -2155,11 +2152,8 @@ spa_vdev_remove(spa_t *spa, uint64_t guid, boolean_t unspare)
 			ev = spa_event_create(spa, vd, NULL,
 			    ESC_ZFS_VDEV_REMOVE_AUX);
 
-			char *nvstr = fnvlist_lookup_string(nv,
-			    ZPOOL_CONFIG_PATH);
-			spa_history_log_internal(spa, "vdev remove", NULL,
-			    "%s vdev (%s) %s", spa_name(spa),
-			    VDEV_TYPE_SPARE, nvstr);
+			vd_type = VDEV_TYPE_SPARE;
+			vd_path = fnvlist_lookup_string(nv, ZPOOL_CONFIG_PATH);
 			spa_vdev_remove_aux(spa->spa_spares.sav_config,
 			    ZPOOL_CONFIG_SPARES, spares, nspares, nv);
 			spa_load_spares(spa);
@@ -2171,9 +2165,8 @@ spa_vdev_remove(spa_t *spa, uint64_t guid, boolean_t unspare)
 	    nvlist_lookup_nvlist_array(spa->spa_l2cache.sav_config,
 	    ZPOOL_CONFIG_L2CACHE, &l2cache, &nl2cache) == 0 &&
 	    (nv = spa_nvlist_lookup_by_guid(l2cache, nl2cache, guid)) != NULL) {
-		char *nvstr = fnvlist_lookup_string(nv, ZPOOL_CONFIG_PATH);
-		spa_history_log_internal(spa, "vdev remove", NULL,
-		    "%s vdev (%s) %s", spa_name(spa), VDEV_TYPE_L2CACHE, nvstr);
+		vd_type = VDEV_TYPE_L2CACHE;
+		vd_path = fnvlist_lookup_string(nv, ZPOOL_CONFIG_PATH);
 		/*
 		 * Cache devices can always be removed.
 		 */
@@ -2185,6 +2178,8 @@ spa_vdev_remove(spa_t *spa, uint64_t guid, boolean_t unspare)
 		spa->spa_l2cache.sav_sync = B_TRUE;
 	} else if (vd != NULL && vd->vdev_islog) {
 		ASSERT(!locked);
+		vd_type = "log";
+		vd_path = (vd->vdev_path != NULL) ? vd->vdev_path : "-";
 		error = spa_vdev_remove_log(vd, &txg);
 	} else if (vd != NULL) {
 		ASSERT(!locked);
@@ -2199,6 +2194,18 @@ spa_vdev_remove(spa_t *spa, uint64_t guid, boolean_t unspare)
 	if (!locked)
 		error = spa_vdev_exit(spa, NULL, txg, error);
 
+	/*
+	 * Logging must be done outside the spa config lock. Otherwise,
+	 * this code path could end up holding the spa config lock while
+	 * waiting for a txg_sync so it can write to the internal log.
+	 * Doing that would prevent the txg sync from actually happening,
+	 * causing a deadlock.
+	 */
+	if (error == 0 && vd_type != NULL && vd_path != NULL) {
+		spa_history_log_internal(spa, "vdev remove", NULL,
+		    "%s vdev (%s) %s", spa_name(spa), vd_type, vd_path);
+	}
+
 	if (ev != NULL)
 		spa_event_post(ev);