Hi,

What you *could* do is to start putting processes to sleep if they
attempt to write to the frozen filesystem, and then detect the
deadlock case where the process holding the file descriptor used to
freeze the filesystem gets frozen because it attempted to write to the
filesystem --- at which point it gets some kind of signal (which
defaults to killing the process), and the filesystem is unfrozen and
as part of the unfreeze you wake up all of the processes that were put
to sleep for touching the frozen filesystem.

The other approach would be to say, "oh well, the freeze ioctl is
inherently dangerous, and root is allowed to himself in the foot, so
who cares".  :-)

Currently the XFS freezer doesn't solve a deadlock automatically
and we rely on administrators for ensuring that the freezer will not
access the filesystem.
And even if the wrong freezer causes a deadlock, it can be solved
by other unfreeze process(unfreeze command).

So I don't think the freezer itself needs to solve the deadlock.
I think the timeout is effective for a unexpected deadlock
and the timeout extending feature is very useful
as Dmitri proposed.

Cheers, Takashi