Add zfs_vdev_parallel_open_enabled module parameter

Parallel vdev open causes the driver to deadlock when running ZFS on top
of a zvol with something in between, including, but not limited to the
loop device. Such configurations are unsupported because we do not have
regression test coverage for them, but enough people have requested the
capability over the years that we ought to give them a knob to make the
configuration work in most instances.

Signed-off-by: Richard Yao <ryao@gentoo.org>

man/man5/zfs-module-parameters.5

@@ -1049,6 +1049,26 @@ See the section "ZFS I/O SCHEDULER".
 Default value: \fB1\fR.
 .RE
 
+.sp
+.ne 2
+.na
+\fBzfs_vdev_parallel_open_enabled\fR (bool)
+.ad
+.RS 12n
+Accelerates vdev open from pool import or reopen from reload from cache by
+allow child vdevs to be opened in parallel.
+.sp
+This almost always deadlocks in unsupported stacked ZFS configurations where a
+zvol from one pool is the vdev for another pool via another type of block
+device. Some logic exists in the kernel module to disable this in the direct
+case when there is nothing in-between, but it is impossible to detect the
+indirect case where another block device exists inbetween. Disabling this will
+often make stacked ZFS configurations work, but they continue to be an
+unsupported configuration because it is subjected to regression testing.
+.sp
+Use \fB1\fR for yes (default) and \fB0\fR for no.
+.RE
+
 .sp
 .ne 2
 .na

module/zfs/vdev.c

@@ -51,6 +51,11 @@
  */
 int metaslabs_per_vdev = 200;
 
+/*
+ * Allow system administrator to disable parallel open optimization so that
+ * unsupported ZFS on something else on zvol is semi-safe.
+ */
+int zfs_vdev_parallel_open_enabled = 1;
 /*
  * Virtual device management.
  */
@@ -1136,7 +1141,7 @@ vdev_uses_zvols(vdev_t *vd)
 	int c;
 
 #ifdef _KERNEL
-	if (zvol_is_zvol(vd->vdev_path))
+	if (zfs_vdev_parallel_open_enabled == 0 || zvol_is_zvol(vd->vdev_path))
 		return (B_TRUE);
 #endif
 
@@ -3642,6 +3647,10 @@ EXPORT_SYMBOL(vdev_online);
 EXPORT_SYMBOL(vdev_offline);
 EXPORT_SYMBOL(vdev_clear);
 
+module_param(zfs_vdev_parallel_open_enabled, int, 0644);
+MODULE_PARM_DESC(zfs_vdev_parallel_open_enabled,
+	"Open vdev children in parallel during import.");
+
 module_param(metaslabs_per_vdev, int, 0644);
 MODULE_PARM_DESC(metaslabs_per_vdev,
 	"Divide added vdev into approximately (but no more than) this number "