Avoid dirtying the final TXGs when exporting a pool

There are two codepaths than can dirty final TXGs:

1) If calling spa_export_common()->spa_unload()->
   spa_unload_log_sm_flush_all() after the spa_final_txg is set, then
   spa_sync()->spa_flush_metaslabs() may end up dirtying the final
   TXGs. Then we have the following panic:
   Call Trace:
    <TASK>
    dump_stack_lvl+0x46/0x62
    spl_panic+0xea/0x102 [spl]
    dbuf_dirty+0xcd6/0x11b0 [zfs]
    zap_lockdir_impl+0x321/0x590 [zfs]
    zap_lockdir+0xed/0x150 [zfs]
    zap_update+0x69/0x250 [zfs]
    feature_sync+0x5f/0x190 [zfs]
    space_map_alloc+0x83/0xc0 [zfs]
    spa_generate_syncing_log_sm+0x10b/0x2f0 [zfs]
    spa_flush_metaslabs+0xb2/0x350 [zfs]
    spa_sync_iterate_to_convergence+0x15a/0x320 [zfs]
    spa_sync+0x2e0/0x840 [zfs]
    txg_sync_thread+0x2b1/0x3f0 [zfs]
    thread_generic_wrapper+0x62/0xa0 [spl]
    kthread+0x127/0x150
    ret_from_fork+0x22/0x30
    </TASK>

2) Calling vdev_*_stop_all() for a second time in spa_unload() after
   spa_export_common() unnecessarily delays the final TXGs beyond what
   spa_final_txg is set at.

Fix this by performing the check and call for
spa_unload_log_sm_flush_all() before the spa_final_txg is set in
spa_export_common(). Also check if the spa_final_txg has already been
set in spa_unload() and skip those calls in this case.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: George Amanakis <gamanakis@gmail.com>
External-issue: https://www.illumos.org/issues/9081
Closes #13048 
Closes #13098

module/zfs/spa.c

@@ -1597,10 +1597,16 @@ spa_unload(spa_t *spa)
 	spa_wake_waiters(spa);
 
 	/*
-	 * If the log space map feature is enabled and the pool is getting
+	 * If we have set the spa_final_txg, we have already performed the
-	 * exported (but not destroyed), we want to spend some time flushing
+	 * tasks below in spa_export_common(). We should not redo it here since
-	 * as many metaslabs as we can in an attempt to destroy log space
+	 * we delay the final TXGs beyond what spa_final_txg is set at.
-	 * maps and save import time.
+	 */
+	if (spa->spa_final_txg == UINT64_MAX) {
+		/*
+		 * If the log space map feature is enabled and the pool is
+		 * getting exported (but not destroyed), we want to spend some
+		 * time flushing as many metaslabs as we can in an attempt to
+		 * destroy log space maps and save import time.
 		 */
 		if (spa_should_flush_logs_on_unload(spa))
 			spa_unload_log_sm_flush_all(spa);
@@ -1612,11 +1618,13 @@ spa_unload(spa_t *spa)
 
 		if (spa->spa_root_vdev) {
 			vdev_t *root_vdev = spa->spa_root_vdev;
-		vdev_initialize_stop_all(root_vdev, VDEV_INITIALIZE_ACTIVE);
+			vdev_initialize_stop_all(root_vdev,
+			    VDEV_INITIALIZE_ACTIVE);
 			vdev_trim_stop_all(root_vdev, VDEV_TRIM_ACTIVE);
 			vdev_autotrim_stop_all(spa);
 			vdev_rebuild_stop_all(spa);
 		}
+	}
 
 	/*
 	 * Stop syncing.
@@ -6427,9 +6435,27 @@ spa_export_common(const char *pool, int new_state, nvlist_t **oldconfig,
 		if (new_state != POOL_STATE_UNINITIALIZED && !hardforce) {
 			spa_config_enter(spa, SCL_ALL, FTAG, RW_WRITER);
 			spa->spa_state = new_state;
+			vdev_config_dirty(spa->spa_root_vdev);
+			spa_config_exit(spa, SCL_ALL, FTAG);
+		}
+
+		/*
+		 * If the log space map feature is enabled and the pool is
+		 * getting exported (but not destroyed), we want to spend some
+		 * time flushing as many metaslabs as we can in an attempt to
+		 * destroy log space maps and save import time. This has to be
+		 * done before we set the spa_final_txg, otherwise
+		 * spa_sync() -> spa_flush_metaslabs() may dirty the final TXGs.
+		 * spa_should_flush_logs_on_unload() should be called after
+		 * spa_state has been set to the new_state.
+		 */
+		if (spa_should_flush_logs_on_unload(spa))
+			spa_unload_log_sm_flush_all(spa);
+
+		if (new_state != POOL_STATE_UNINITIALIZED && !hardforce) {
+			spa_config_enter(spa, SCL_ALL, FTAG, RW_WRITER);
 			spa->spa_final_txg = spa_last_synced_txg(spa) +
 			    TXG_DEFER_SIZE + 1;
-			vdev_config_dirty(spa->spa_root_vdev);
 			spa_config_exit(spa, SCL_ALL, FTAG);
 		}
 	}