Peter Holm reports that his test that does an umount(8) on an active
mount point while numerous tests are running that are writing to
files on that mount point cause the unmount(8) to hang forever.

The unmount(8) system call is handled in the kernel by the dounmount()
function. The cause of the hang is that prior to dounmount() calling
VFS_UNMOUNT() it is calling VFS_SYNC(mp, MNT_WAIT). The MNT_WAIT
flag indicates that VFS_SYNC() should not return until all the dirty
buffers associated with the mount point have been written to disk.
Because user processes are allowed to continue writing and can do
so faster than the data can be written to disk, the call to VFS_SYNC()
can never finish.

Unlike VFS_SYNC(), the VFS_UNMOUNT() routine can suspend all processes
when they request to do a write thus having a finite number of dirty
buffers to write that cannot be expanded. There is no need to call
VFS_SYNC() before calling VFS_UNMOUNT(), because VFS_UNMOUNT() needs
to flush everything again anyway after suspending writes, to catch
anything that was dirtied between the VFS_SYNC() and writes being
suspended.

The fix is to simply remove the unnecessary call to VFS_SYNC() from
dounmount().

Reported by: Peter Holm
Analysis by: Chuck Silvers
Tested by: Peter Holm
MFC after: 7 days
Sponsored by: Netflix