Fix self-healing IO prior to dsl_pool_init() completion

Async writes triggered by a self-healing IO may be issued before the
pool finishes the process of initialization.  This results in a NULL
dereference of `spa->spa_dsl_pool` in vdev_queue_max_async_writes().

George Wilson recommended addressing this issue by initializing the
passed `dsl_pool_t **` prior to dmu_objset_open_impl().  Since the
caller is passing the `spa->spa_dsl_pool` this has the effect of
ensuring it's initialized.

However, since this depends on the caller knowing they must pass
the `spa->spa_dsl_pool` an additional NULL check was added to
vdev_queue_max_async_writes().  This guards against any future
restructuring of the code which might result in dsl_pool_init()
being called differently.

Signed-off-by: GeLiXin <47034221@qq.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #4652

module/zfs/dsl_pool.c

@@ -182,12 +182,20 @@ dsl_pool_init(spa_t *spa, uint64_t txg, dsl_pool_t **dpp)
 	int err;
 	dsl_pool_t *dp = dsl_pool_open_impl(spa, txg);
 
+	/*
+	 * Initialize the caller's dsl_pool_t structure before we actually open
+	 * the meta objset.  This is done because a self-healing write zio may
+	 * be issued as part of dmu_objset_open_impl() and the spa needs its
+	 * dsl_pool_t initialized in order to handle the write.
+	 */
+	*dpp = dp;
+
 	err = dmu_objset_open_impl(spa, NULL, &dp->dp_meta_rootbp,
 	    &dp->dp_meta_objset);
-	if (err != 0)
+	if (err != 0) {
 		dsl_pool_close(dp);
-	else
-		*dpp = dp;
+		*dpp = NULL;
+	}
 
 	return (err);
 }

module/zfs/vdev_queue.c

@@ -249,20 +249,29 @@ static int
 vdev_queue_max_async_writes(spa_t *spa)
 {
 	int writes;
-	uint64_t dirty = spa->spa_dsl_pool->dp_dirty_total;
+	uint64_t dirty = 0;
+	dsl_pool_t *dp = spa_get_dsl(spa);
 	uint64_t min_bytes = zfs_dirty_data_max *
 	    zfs_vdev_async_write_active_min_dirty_percent / 100;
 	uint64_t max_bytes = zfs_dirty_data_max *
 	    zfs_vdev_async_write_active_max_dirty_percent / 100;
 
+	/*
+	 * Async writes may occur before the assignment of the spa's
+	 * dsl_pool_t if a self-healing zio is issued prior to the
+	 * completion of dmu_objset_open_impl().
+	 */
+	if (dp == NULL)
+		return (zfs_vdev_async_write_max_active);
+
 	/*
 	 * Sync tasks correspond to interactive user actions. To reduce the
 	 * execution time of those actions we push data out as fast as possible.
 	 */
-	if (spa_has_pending_synctask(spa)) {
+	if (spa_has_pending_synctask(spa))
 		return (zfs_vdev_async_write_max_active);
-	}
 
+	dirty = dp->dp_dirty_total;
 	if (dirty < min_bytes)
 		return (zfs_vdev_async_write_min_active);
 	if (dirty > max_bytes)