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]

[Rick Macklem]

Harry Schmalzbauer <freebsd at omnilan.de> wrote:
>Bezüglich Rick Macklem's Nachricht vom 18.08.2016 02:03 (localtime):
>>  Kostik wrote:
>> [stuff snipped]
>>> insmnque() performs the cleanup on its own, and that default cleanup isnot suitable >for the situation.  I think that insmntque1() would betterfit your requirements, your >need to move the common code into a helper.It seems that >unionfs_ins_cached_vnode() cleanup could reuse it.
>> <https://lists.freebsd.org>
>> I've attached an updated patch (untested like the last one). This one creates a
>> custom version insmntque_stddtr() that first calls unionfs_noderem() and then
>> does the same stuff as insmntque_stddtr(). This looks like it does the required
>> stuff (unionfs_noderem() is what the unionfs VOP_RECLAIM() does).
>> It switches the node back to using its own v_vnlock that is exclusively locked,
>> among other things.
>
>Thanks a lot, today I gave it a try.
>
>With this patch, one reproducable panic can still be easily triggered:
>        I have directory A unionfs_mounted under directory B.
>Then I mount_unionfs the same directory A below another directory C.
>panic: __lockmgr_args: downgrade a recursed lockmgr nfs @
>/usr/local/share/deploy-tools/RELENG_11/src/sys/fs/unionfs/union_vnops.c:1905
>Result is this backtrace, hardly helpful I guess:
>
>#1  0xffffffff80ae5fd9 in kern_reboot (howto=260) at
>/usr/local/share/deploy-tools/RELENG_11/src/sys/kern/kern_shutdown.c:366
>#2  0xffffffff80ae658b in vpanic (fmt=<value optimized out>, ap=<value
>optimized out>)
>    at
>/usr/local/share/deploy-tools/RELENG_11/src/sys/kern/kern_shutdown.c:759
>#3  0xffffffff80ae63c3 in panic (fmt=0x0) at
>/usr/local/share/deploy-tools/RELENG_11/src/sys/kern/kern_shutdown.c:690
>#4  0xffffffff80ab7ab7 in __lockmgr_args (lk=<value optimized out>,
>flags=<value optimized out>, ilk=<value optimized out>, wmesg=<value
>optimized out>,
>    pri=<value optimized out>, timo=<value optimized out>, file=<value
>optimized out>, line=<value optimized out>)
>  >   at /usr/local/share/deploy-tools/RELENG_11/src/sys/kern/kern_lock.c:992
>#5  0xffffffff80ba510c in vop_stdlock (ap=<value optimized out>) at
>lockmgr.h:98
>#6  0xffffffff8111932d in VOP_LOCK1_APV (vop=<value optimized out>,
>a=<value optimized out>) at vnode_if.c:2087
>#7  0xffffffff80a18cfc in unionfs_lock (ap=0xfffffe007a3ba6a0) at
>vnode_if.h:859
>#8  0xffffffff8111932d in VOP_LOCK1_APV (vop=<value optimized out>,
>a=<value optimized out>) at vnode_if.c:2087
>#9  0xffffffff80bc9b93 in _vn_lock (vp=<value optimized out>,
>flags=66560, file=<value optimized out>, line=<value optimized out>) at
>vnode_if.h:859
>#10 0xffffffff80a18460 in unionfs_readdir (ap=<value optimized out>) at
>/usr/local/share/deploy-tools/RELENG_11/src/sys/fs/unionfs/union_vnops.c:1531
>#11 0xffffffff81118ecf in VOP_READDIR_APV (vop=<value optimized out>,
>a=<value optimized out>) at vnode_if.c:1822
>#12 0xffffffff80bc6e3b in kern_getdirentries (td=<value optimized out>,
>fd=<value optimized out>, buf=0x800c3d000 <Address 0x800c3d000 out of
>bounds>,
>    count=<value optimized out>, basep=0xfffffe007a3ba980, residp=0x0)
>at vnode_if.h:758
>#13 0xffffffff80bc6bf8 in sys_getdirentries (td=0x0,
>uap=0xfffffe007a3baa40) at
>/usr/local/share/deploy-tools/RELENG_11/src/sys/kern/vfs_syscalls.c:3940
>#14 0xffffffff80fad6b8 in amd64_syscall (td=<value optimized out>,
>traced=0) at subr_syscall.c:135
>#15 0xffffffff80f8feab in Xfast_syscall () at
>/usr/local/share/deploy-tools/RELENG_11/src/sys/amd64/amd64/exception.S:396
>#16 0x0000000000452eea in ?? ()
>Previous frame inner to this frame (corrupt stack?
Ok, I finally got around to looking at this and the panic() looks like a pretty straightforward
bug in the unionfs code.
- In unionfs_readdir(), it does a vn_lock(..LK_UPGRADE) and then later in the code
  vn_lock(..LK_DOWNGRADE) if it did the upgrade. (At line#1531 as noted in the backtrace.)
  - In unionfs_lock(), it sets LK_CANRECURSE when it is the rootvp and LK_EXCLUSIVE.
   (So it allows recursive acquisition in this case.)
--> Then it would call vn_lock(..LK_DOWNGRADE), which would panic if it has recursed.

Now, I'll admit unionfs_lock() is too obscure for me to understand, but...
Is it necessary to vn_lock(..LK_DOWNGRADE) or can unionfs_readdir() just return
with the vnode exclusively locked?
(It would be easy to change the code to avoid the vn_lock(..LK_DOWNGRADE) call
 when it has done the vn_lock(..LK_EXCLUSIVE) after vn_lock(..LK_UPGRADE) fails.)

rick

>I ran your previous patch with for some time.
>Similarly, mounting one directory below a 2nd mountpount crashed the
>machine (forgot to config dumpdir, so can't compare backtrace with the
>current patch).
>Otherwise, at least with the previous patch, I haven't had any other
>panic for about one week.
>
>Thanks,
>
>-Harry