try to fix lor between z_teardown_lock and spa_namespace_lock

The lock order reversal and a resulting deadlock were introduced
in r285021 / D2865.  The problem is that zfs_register_callbacks() calls
dsl_prop_get_integer() that has to acquire spa_namespace_lock.
At the same time, spa_config_sync() is called with spa_namespace_lock
held and then it performs ZFS vnode operations that acquire
z_teardown_lock in the reader mode.

So, fix the problem by using dsl_prop_get_int_ds() instead of
dsl_prop_get_integer().  The former does not need to look up
the pool and the dataset by name.

Reported by:	many
Reviewed by:	delphij
Tested by:	delphij, Jens Schweikhardt <schweikh@schweikhardt.net>
MFC after:	5 days
X-MFC with:	r285021

1 files changed, 16 insertions, 10 deletions

diff --git a/sys/cddl/contrib/opensolaris/uts/common/fs/zfs/zfs_vfsops.c b/sys/cddl/contrib/opensolaris/uts/common/fs/zfs/zfs_vfsops.c
index ed56d17..b45d958 100644
--- a/sys/cddl/contrib/opensolaris/uts/common/fs/zfs/zfs_vfsops.c
+++ b/sys/cddl/contrib/opensolaris/uts/common/fs/zfs/zfs_vfsops.c
@@ -472,6 +472,19 @@ zfs_register_callbacks(vfs_t *vfsp)
 	}
 
 	/*
+	 * We need to enter pool configuration here, so that we can use
+	 * dsl_prop_get_int_ds() to handle the special nbmand property below.
+	 * dsl_prop_get_integer() can not be used, because it has to acquire
+	 * spa_namespace_lock and we can not do that because we already hold
+	 * z_teardown_lock.  The problem is that spa_config_sync() is called
+	 * with spa_namespace_lock held and the function calls ZFS vnode
+	 * operations to write the cache file and thus z_teardown_lock is
+	 * acquired after spa_namespace_lock.
+	 */
+	ds = dmu_objset_ds(os);
+	dsl_pool_config_enter(dmu_objset_pool(os), FTAG);
+
+	/*
 	 * nbmand is a special property.  It can only be changed at
 	 * mount time.
 	 *
@@ -482,14 +495,9 @@ zfs_register_callbacks(vfs_t *vfsp)
 		nbmand = B_FALSE;
 	} else if (vfs_optionisset(vfsp, MNTOPT_NBMAND, NULL)) {
 		nbmand = B_TRUE;
-	} else {
-		char osname[MAXNAMELEN];
-
-		dmu_objset_name(os, osname);
-		if (error = dsl_prop_get_integer(osname, "nbmand", &nbmand,
-		    NULL)) {
-			return (error);
-		}
+	} else if (error = dsl_prop_get_int_ds(ds, "nbmand", &nbmand) != 0) {
+		dsl_pool_config_exit(dmu_objset_pool(os), FTAG);
+		return (error);
 	}
 
 	/*
@@ -499,8 +507,6 @@ zfs_register_callbacks(vfs_t *vfsp)
 	 * the first prop_register(), but I guess I like to go
 	 * overboard...
 	 */
-	ds = dmu_objset_ds(os);
-	dsl_pool_config_enter(dmu_objset_pool(os), FTAG);
 	error = dsl_prop_register(ds,
 	    zfs_prop_to_name(ZFS_PROP_ATIME), atime_changed_cb, zfsvfs);
 	error = error ? error : dsl_prop_register(ds,