dm ioctl: make bio or request based device type immutable

Determine whether a mapped device is bio-based or request-based when
loading its first (inactive) table and don't allow that to be changed
later.

This patch performs different device initialisation in each of the two
cases.  (We don't think it's necessary to add code to support changing
between the two types.)

Allowed md->type transitions:
  DM_TYPE_NONE to DM_TYPE_BIO_BASED
  DM_TYPE_NONE to DM_TYPE_REQUEST_BASED

We now prevent table_load from replacing the inactive table with a
conflicting type of table even after an explicit table_clear.

Introduce 'type_lock' into the struct mapped_device to protect md->type
and to prepare for the next patch that will change the queue
initialization and allocate memory while md->type_lock is held.

Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Acked-by: Kiyoshi Ueda <k-ueda@ct.jp.nec.com>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>

 drivers/md/dm-ioctl.c    |   15 +++++++++++++++
 drivers/md/dm.c          |   37 ++++++++++++++++++++++++++++++-------
 drivers/md/dm.h          |    5 +++++
 include/linux/dm-ioctl.h |    4 ++--
 4 files changed, 52 insertions(+), 9 deletions(-)

1 files changed, 15 insertions, 0 deletions

diff --git a/drivers/md/dm-ioctl.c b/drivers/md/dm-ioctl.c
index 3fd8f0e169e..4702f380cb4 100644
--- a/drivers/md/dm-ioctl.c
+++ b/drivers/md/dm-ioctl.c
@@ -1189,6 +1189,21 @@ static int table_load(struct dm_ioctl *param, size_t param_size)
 		goto out;
 	}
 
+	/* Protect md->type against concurrent table loads. */
+	dm_lock_md_type(md);
+	if (dm_get_md_type(md) == DM_TYPE_NONE)
+		/* Initial table load: acquire type of table. */
+		dm_set_md_type(md, dm_table_get_type(t));
+	else if (dm_get_md_type(md) != dm_table_get_type(t)) {
+		DMWARN("can't change device type after initial table load.");
+		dm_table_destroy(t);
+		dm_unlock_md_type(md);
+		r = -EINVAL;
+		goto out;
+	}
+	dm_unlock_md_type(md);
+
+	/* stage inactive table */
 	down_write(&_hash_lock);
 	hc = dm_get_mdptr(md);
 	if (!hc || hc->md != md) {