module param callbacks check for initialized spa

Callbacks provided for module parameters are executed both
after the module is loaded, when a user alters it via sysfs, e.g
	echo bar > /sys/modules/zfs/parameters/foo

as well as when the module is loaded with an argument, e.g.
	modprobe zfs foo=bar

In the latter case, the init functions likely have not run yet,
including spa_init() which initializes the namespace lock so it is safe
to use.

Instead of immediately taking the namespace lock and attemping to
iterate over initialized spa structures, check whether spa_mode_global
is nonzero.  This is set by spa_init() after it has initialized the
namespace lock.

Reviewed-by: Brian Behlendorf <[email protected]>
Reviewed-by: Tim Chase <[email protected]>
Signed-off-by: Olaf Faaland <[email protected]>
Closes #7496 
Closes #7521 

1 files changed, 13 insertions, 11 deletions

diff --git a/module/zfs/vdev_disk.c b/module/zfs/vdev_disk.c
index f537c0d27..9c3a1ba80 100644
--- a/module/zfs/vdev_disk.c
+++ b/module/zfs/vdev_disk.c
@@ -818,19 +818,21 @@ param_set_vdev_scheduler(const char *val, zfs_kernel_param_t *kp)
 	if ((p = strchr(val, '\n')) != NULL)
 		*p = '\0';
 
-	mutex_enter(&spa_namespace_lock);
-	while ((spa = spa_next(spa)) != NULL) {
-		if (spa_state(spa) != POOL_STATE_ACTIVE ||
-		    !spa_writeable(spa) || spa_suspended(spa))
-			continue;
-
-		spa_open_ref(spa, FTAG);
-		mutex_exit(&spa_namespace_lock);
-		vdev_elevator_switch(spa->spa_root_vdev, (char *)val);
+	if (spa_mode_global != 0) {
 		mutex_enter(&spa_namespace_lock);
-		spa_close(spa, FTAG);
+		while ((spa = spa_next(spa)) != NULL) {
+			if (spa_state(spa) != POOL_STATE_ACTIVE ||
+			    !spa_writeable(spa) || spa_suspended(spa))
+				continue;
+
+			spa_open_ref(spa, FTAG);
+			mutex_exit(&spa_namespace_lock);
+			vdev_elevator_switch(spa->spa_root_vdev, (char *)val);
+			mutex_enter(&spa_namespace_lock);
+			spa_close(spa, FTAG);
+		}
+		mutex_exit(&spa_namespace_lock);
 	}
-	mutex_exit(&spa_namespace_lock);
 
 	return (param_set_charp(val, kp));
 }