debugging process in bovlbx state

[John Baldwin]

On Sunday, December 19, 2010 7:10:04 pm Benjamin Kaduk wrote:
> Hi all,
> 
> I'm working on bringing the out-of-tree OpenAFS network filesystem 
> up-to-date for FreeBSD 7.3-RELEASE, and I think I need some help to fix 
> this bug.
> I should preface my discourse with the fact that there is a whole slow of 
> lock order reversals that I haven't even tried to track down, but I do not 
> believe that this hang is deadlock since 'show alllocks' in DDB does not 
> show anything that seems interesting.
> 
> Any pointers for things to look at would be appreciated; more details of 
> the failing case below.
> 
> 
> In order to get the afs kernel module to load, I needed to tweak a few 
> lines of code in getpages(), as I had previously cribbed a bunch of 
> changes/updates from the experimental NFS client while getting AFS to work 
> on current freebsd.  In particular, vm_page_set_valid is not present in 
> 7.3, so I am currently running with:
> --- a/src/afs/FBSD/osi_vnodeops.c
> +++ b/src/afs/FBSD/osi_vnodeops.c
> @@ -890,12 +890,8 @@ afs_vop_getpages(struct vop_getpages_args *ap)
>                * Read operation filled a partial page.
>                */
>               m->valid = 0;
> -           vm_page_set_valid(m, 0, size - toff);
> -#ifndef AFS_FBSD80_ENV
> -           vm_page_undirty(m);
> -#else
> +           vm_page_set_validclean(m, 0, size - toff);
>               KASSERT(m->dirty == 0, ("afs_getpages: page %p is dirty", m));
> -#endif
>           }
> 
> 
> But my knowledge of vm_page_* is approximately nil, so there's no reason 
> to think everything was correct even before that patch.
> 
> Anyway, my test case is running libarchive's configure script with source 
> and destination directories in (different places in) AFS.  It only gets 
> twenty lines in, ending with:
> checking for gcc option to accept ISO C89... none needed
> checking for style of include used by make... GNU
> checking dependency style of gcc...
> ^Tload: 0.04 cmd: cp 1250 [bovlbx] 0.00u 0.00
> 
> procstat -kk reports:
> mega-man# procstat -kk 1250
>     PID    TID COMM             TDNAME           KSTACK
>    1250 100060 cp               -                mi_switch+0x233 
> sleepq_switch+0xe9 sleepq_wait+0x44 _sleep+0x3a0 vm_object_pip_wait+0x4e 
> bufobj_invalbuf+0x10e afs_GetVCache+0x2f7
> 
> The call to vinvalbuf in afs_GetVCache is here:
>      1646         iheldthelock = VOP_ISLOCKED(vp, curthread);

This is probably wrong.  VOP_ISLOCKED() can return four different values:

- LK_SHARED: (someone, possibly curthread) holds a shared lock
- LK_EXCLUSIVE: curthread holds an exclusive lock
- LK_EXCLOTHER: some other thread holds an exclusive lock
- 0: no thread holds any lock.

This means if another thread has the vnode locked, you don't try to lock it. 
:)

Do you actually know that this routine can be held without the vnode locked by 
the current thread?

-- 
John Baldwin