Fix zfs_sb_teardown/zfs_resume_fs NULL dereference

Fix a pair of conditions in which a concurrent umount can cause
NULL pointer dereferences:

* zfs_sb_teardown - prevent a NULL dereference by not calling
                    dmu_objset_pool with a null z_os.

* zfs_resume_fs - don't try to unmount with a null z_os.  This
                  change makes the ZoL code more consistent
                  with both Illumos and FreeBSD.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #1543

diff --git a/module/zfs/zfs_vfsops.c b/module/zfs/zfs_vfsops.c
index e618f2b..602c332 100644 (file)
--- a/module/zfs/zfs_vfsops.c
+++ b/module/zfs/zfs_vfsops.c
@@ -1056,10 +1056,12 @@ zfs_sb_teardown(zfs_sb_t *zsb, boolean_t unmounting)
        }
 
        /*
-        * Drain the iput_taskq to ensure all active references to the
+        * If someone has not already unmounted this file system,
+        * drain the iput_taskq to ensure all active references to the
         * zfs_sb_t have been handled only then can it be safely destroyed.
         */
-       taskq_wait(dsl_pool_iput_taskq(dmu_objset_pool(zsb->z_os)));
+       if (zsb->z_os)
+               taskq_wait(dsl_pool_iput_taskq(dmu_objset_pool(zsb->z_os)));
 
        /*
         * Close the zil. NB: Can't close the zil while zfs_inactive
@@ -1480,10 +1482,11 @@ bail:
 
        if (err) {
                /*
-                * Since we couldn't reopen zfs_sb_t, force
-                * unmount this file system.
+                * Since we couldn't reopen zfs_sb_t or, setup the
+                * sa framework, force unmount this file system.
                 */
-               (void) zfs_umount(zsb->z_sb);
+               if (zsb->z_os)
+                       (void) zfs_umount(zsb->z_sb);
        }
        return (err);
 }