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 Harry Schmalzbauer's Nachricht vom 07.03.2017 19:44 (localtime):
>  Bezüglich Harry Schmalzbauer's Nachricht vom 07.03.2017 13:42 (localtime):
> …
>> Something ufs related seems to have tightened the unionfs locking
>> problem in stable/11.  Now the machine instantaniously panics during
>> boot after mounting root with Rick's latest patch.
>>
>> Unfortunately I don't have SWAP available on that machine (yet), but
>> maybe shit is a hint for anybody.
>>
>> db_trace_self_wrapper() at db_trace_self_wrapper+0x2b/frame
>> 0xfffffe00982220e0
>> vpanic() at vpanic+0x186/frame 0xfffffe0098222160
>> kassert_panic() at kassert_panic+0x126/frame 0xfffffe00982221d0
>> witness_assert() at witness_assert+0x35a/frame 0xfffffe0098222230
>> __lockmgr_args() at __lockmgr_args+0x517/frame 0xfffffe00982222d0
>> vop_stdunlock() at vop_stdunlock+0x3b/frame 0xfffffe00982222f0
>> VOP_UNLOCK_APV() at VOP_UNLOCK_APV+0xe0/frame 0xfffffe0098222320
>> unionfs_unlock() at unionfs_unlock+0x112/frame 0xfffffe0098222390
>> VOP_UNLOCK_APV() at VOP_UNLOCK_APV+0xe0/frame 0xfffffe00982223c0
>> unionfs_nodeget() at unionfs_nodeget+0x3ef/frame 0xfffffe0098222470
>> unionfs_domount() at unionfs_domount+0x518/frame 0xfffffe00982226b0
>> vfs_donmount() at vfs_donmount+0xe37/frame 0xfffffe00982228f0
>> sys_nmount() at sys_nmount+0x72/frame 0xfffffe0098222930
>> amd64_syscall() at amd64_syscall+0x2f9/frame 0xfffffe0098222ab0
>> Xfast_syscall() at Xfast_syscall+0xfb/frame 0xfffffe0098222ab0
>> --- syscall (378, FreeBSD ELF64, sys_nmount), rip = 0x80086ecea, rsp =
>> 0x7fffffffe318, rbp = 0x7fffffffeca0 ---
> New discovery:
> Rick's latest patch casues panic only with KDB. If I compile a kernel
> without witenss and KDB, the machine boots fine!
> Also, it's at least not so easy anymore to trigger the deadlock :-) . I
> need to do more testing but until now Rick's approach seems very
> promising :-) . 

My unionfs deadlock problem isn't really solved with Rick's latest
patch, I still can reproduce it: krb5.conf and krb5.keytab are files on
unionfs referenced by /etc.  libexec/negotiate_kerberos_auth reads these
and if I have enough helper processes handling requests, the deadlock
occurs.

_But_: If I move the files outside the unionfs and create a symlink, I
cannot reproduce the deadlock anymore, which was similar easily
reproducable without it or any of the other workarounds.
So it looks like I have an acceptable solution for now, although it's
only usable under certain conditions.

Unfortunately I can't do tests with a debug kernel since the patch
prevents the system with the debug kernel from starting up.
But if this was ironed out, I'd happily provide more info.


Thanks,

-Harry