I'm seeing some failures running xfstests on cifs when it comes to tests that
do a shutdown of the filesystem, generic/392 for example:

generic/392       [failed, exit status 1]- output mismatch (see /root/xfstests-dev/results//smb3/generic/392.out.bad)
    --- tests/generic/392.out   2021-05-25 13:27:50.000000000 +0100
    +++ /root/xfstests-dev/results//smb3/generic/392.out.bad    2023-04-27 09:07:42.402657080 +0100
    @@ -1,11 +1,67 @@
     QA output created by 392
     ==== i_size 1024 test with fsync ====
    +stat: cannot statx '/xfstest.scratch/testfile': Input/output error
    +Before:  "b: 8194 s: 4195328 a: 2023-04-27 09:07:39.410675400 +0100 m: 2023-04-27 09:07:39.410675400 +0100 c: 2023-04-27 09:07:39.410675400 +0100"
    +After : 
    +rm: cannot remove '/xfstest.scratch/testfile': Input/output error
     ==== i_size 4096 test with fsync ====
    ...
    (Run 'diff -u /root/xfstests-dev/tests/generic/392.out /root/xfstests-dev/results//smb3/generic/392.out.bad'  to see the entire diff)

The problem appears to be that the CIFS_MOUNT_SHUTDOWN persists if there's
another cifs mount from the same server present.  So in generic/392 it does:

	before=`stat "$stat_opt" $testfile`

	$XFS_IO_PROG -c "$sync_mode" $testfile | _filter_xfs_io
	_scratch_shutdown | tee -a $seqres.full
	_scratch_cycle_mount

	after=`stat "$stat_opt" $testfile`

which cycles the *scratch* mount, but leaves the test mount still mounted and
then the 'after' stat fails with EIO.

Testing this by hand:

  mount //192.168.6.1/scratch /xfstest.scratch/ -o user=shares,pass=...;\
  touch /xfstest.scratch/testfile; \
  stat /xfstest.scratch/testfile; \
  ./src/godown /xfstest.scratch/; \
  umount /xfstest.scratch/; \
  mount //192.168.6.1/scratch /xfstest.scratch/ -o user=shares,pass=...; \
  stat /xfstest.scratch/testfile

works, but will fail if I do:

  mount //192.168.6.1/test /xfstest.test/ -o user=shares,pass=...;

first.

Interestingly, the two mounts have different device numbers according to stat,
so they would appear to have different superblocks.

David