quoted

Yes but in the unlikely event that this happens, the effect is a small
memory leak for the duration of the mount.  On the other hand if the
fid is destroyed without successfully informing the server, then
subsequent operations that involve new file references will fail
when that fid number is reused, and the mount becomes unusable.

I don't know whether Sasha's problem is caused by this patch or not.
But p9_client_clunk() is called from many functions in fs/9p/ directory.
They are assuming that p9_client_clunk() will call p9_fid_destroy() but
this patch is breaking that assumption. I think this is the cause of hang which
Sasha is experiencing because Sasha's trace shows that call_usermodehelper() is
blocked by functions in fs/9p/ directory. Seems inconsistency state problem.

I'd be happy to try out any other patches or help debugging this issue.

Which behavior did this patch fix exactly? can I just revert it and
try running without it?