Fix z_xattr_lock/z_teardown_lock lock inversion

There exists a lock inversion between the z_xattr_lock and the
z_teardown_lock.  Detect this case and return EBUSY so zfs_resume_fs()
will mark the inode stale and it can be safely revalidated on next
access.

* process-1
zpl_xattr_get -> Takes zp->z_xattr_lock
  __zpl_xattr_get
    zfs_lookup -> Takes zsb->z_teardown_lock in ZFS_ENTER macro

* process-2
zfs_ioc_recv -> Takes zsb->z_teardown_lock in zfs_suspend_fs()
  zfs_resume_fs
    zfs_rezget -> Takes zp->z_xattr_lock

Signed-off-by: Brian Behlendorf <[email protected]>
Signed-off-by: Chunwei Chen <[email protected]>
Closes #3969

1 files changed, 10 insertions, 1 deletions

diff --git a/module/zfs/zfs_znode.c b/module/zfs/zfs_znode.c
index bf360f1bc..6fda78e5f 100644
--- a/module/zfs/zfs_znode.c
+++ b/module/zfs/zfs_znode.c
@@ -1012,7 +1012,16 @@ zfs_rezget(znode_t *zp)
 	}
 	mutex_exit(&zp->z_acl_lock);
 
-	rw_enter(&zp->z_xattr_lock, RW_WRITER);
+	/*
+	 * Lock inversion with zpl_xattr_get->__zpl_xattr_get->zfs_lookup
+	 * between z_xattr_lock and z_teardown_lock.  Detect this case and
+	 * return EBUSY so zfs_resume_fs() will mark the inode stale and it
+	 * will safely be revalidated on next access.
+	 */
+	err = rw_tryenter(&zp->z_xattr_lock, RW_WRITER);
+	if (!err)
+		return (SET_ERROR(EBUSY));
+
 	if (zp->z_xattr_cached) {
 		nvlist_free(zp->z_xattr_cached);
 		zp->z_xattr_cached = NULL;