unionfs bugs, a partial patch and some comments [Was: Re: 1-BETA3 Panic: __lockmgr_args: downgrade a recursed lockmgr nfs @ /usr/local/share/deploy-tools/RELENG_11/src/sys/fs/unionfs/union_vnops.c:1905]

[Harry Schmalzbauer]

Bezüglich Mark Johnston's Nachricht vom 09.08.2016 08:02 (localtime):
…
>>
>> Just for anybody else needing unionfs:
>> https://people.freebsd.org/~attilio/unionfs_missing_insmntque_lock.patch
>>
>> This patch still applies and I'm successfully using this (unmodified) up
>> to FreeBSD-10.3 and never had any panic in all these years.
> 
> Having spent some time looking at unionfs, I'm a bit skeptical that this
> patch will address the panic you reported earlier, though I'd be
> interested to know if it does. 

Thanks for your attention.
I can confirm that it has prevented panics for more than 4 years
(9.0-10.3) and it seems to be still "good enough" to also prevent panics
in 11-BETA4.
I updated my build host (stable/11, this time with the
unionfs_missing_insmntque_lock.patch), where the recent panics happened
and unionfs gets much more utilized than usually in my setups: No panic
with that patch anymore.
Just one message like "prevented resource deadlock" occured.

> Reading the code, I think it will just
> address an INVARIANTS-only assertion in insmntque1().
> 
> Unfortunately, unionfs is quite difficult to fix within the current
> constraints of FreeBSD's VFS. unionfs_readdir() is a particularly good
> demonstration of this fact: some callers of VOP_READDIR expect the
> cookies returned by the FS to be monotonically increasing, but unionfs
> has no straightforward way to make this guarantee.

I'm sorry, I can't provide help here. My skills would require a huge
ammount of lerning-time to get into that matter. I'd love to do that,
but I can't afford :-(

Thanks,

-Harry