dm: path selector use module refcount directly

Fix refcount corruption in dm-path-selector

Refcounting with non-atomic ops under shared lock will corrupt the counter
in multi-processor system and may trigger BUG_ON().
Use module refcount.
# same approach as dm-target-use-module-refcount-directly.patch here
# https://www.redhat.com/archives/dm-devel/2008-December/msg00075.html

Typical oops:
  kernel BUG at linux-2.6.29-rc3/drivers/md/dm-path-selector.c:90!
  Pid: 11148, comm: dmsetup Not tainted 2.6.29-rc3-nm #1
  dm_put_path_selector+0x4d/0x61 [dm_multipath]
  Call Trace:
   [<ffffffffa031d3f9>] free_priority_group+0x33/0xb3 [dm_multipath]
   [<ffffffffa031d4aa>] free_multipath+0x31/0x67 [dm_multipath]
   [<ffffffffa031d50d>] multipath_dtr+0x2d/0x32 [dm_multipath]
   [<ffffffffa015d6c2>] dm_table_destroy+0x64/0xd8 [dm_mod]
   [<ffffffffa015b73a>] __unbind+0x46/0x4b [dm_mod]
   [<ffffffffa015b79f>] dm_swap_table+0x60/0x14d [dm_mod]
   [<ffffffffa015f963>] dev_suspend+0xfd/0x177 [dm_mod]
   [<ffffffffa0160250>] dm_ctl_ioctl+0x24c/0x29c [dm_mod]
   [<ffffffff80288cd3>] ? get_page_from_freelist+0x49c/0x61d
   [<ffffffffa015f866>] ? dev_suspend+0x0/0x177 [dm_mod]
   [<ffffffff802bf05c>] vfs_ioctl+0x2a/0x77
   [<ffffffff802bf4f1>] do_vfs_ioctl+0x448/0x4a0
   [<ffffffff802bf5a0>] sys_ioctl+0x57/0x7a
   [<ffffffff8020c05b>] system_call_fastpath+0x16/0x1b

Cc: stable@kernel.org
Signed-off-by: Jun'ichi Nomura <j-nomura@ce.jp.nec.com>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>

drivers/md/dm-path-selector.c

@@ -17,9 +17,7 @@
 
 struct ps_internal {
 	struct path_selector_type pst;
-
 	struct list_head list;
-	long use;
 };
 
 #define pst_to_psi(__pst) container_of((__pst), struct ps_internal, pst)
@@ -45,12 +43,8 @@ static struct ps_internal *get_path_selector(const char *name)
 
 	down_read(&_ps_lock);
 	psi = __find_path_selector_type(name);
-	if (psi) {
-		if ((psi->use == 0) && !try_module_get(psi->pst.module))
-			psi = NULL;
-		else
-			psi->use++;
-	}
+	if (psi && !try_module_get(psi->pst.module))
+		psi = NULL;
 	up_read(&_ps_lock);
 
 	return psi;
@@ -84,11 +78,7 @@ void dm_put_path_selector(struct path_selector_type *pst)
 	if (!psi)
 		goto out;
 
-	if (--psi->use == 0)
-		module_put(psi->pst.module);
-
-	BUG_ON(psi->use < 0);
-
+	module_put(psi->pst.module);
 out:
 	up_read(&_ps_lock);
 }
@@ -136,11 +126,6 @@ int dm_unregister_path_selector(struct path_selector_type *pst)
 		return -EINVAL;
 	}
 
-	if (psi->use) {
-		up_write(&_ps_lock);
-		return -ETXTBSY;
-	}
-
 	list_del(&psi->list);
 
 	up_write(&_ps_lock);