On Sat, Feb 15, 2014 at 02:12:40PM +0200, Andriy Gapon wrote:
> on 14/02/2014 21:18 Jeremie Le Hen said the following:
> > I've just got another occurence of the exact same panic.  Any clue how
> > to debug this?
> Could you please obtain *vp from frame 12 ?
> The problem seems to be happening in this piece of ZFS code:
>                 if (cnp->cn_flags & ISDOTDOT) {
>                         ltype = VOP_ISLOCKED(dvp);
>                         VOP_UNLOCK(dvp, 0);
>                 }
>                 ZFS_EXIT(zfsvfs);
>                 error = vn_lock(*vpp, cnp->cn_lkflags);
>                 if (cnp->cn_flags & ISDOTDOT)
>                         vn_lock(dvp, ltype | LK_RETRY);
> ltype is apparently LK_SHARED and the assertion is apparently triggered by
> EDEADLK error.  The error can occur only if a thread tries to obtain a lock 
> in a
> shared mode when it already has the lock exclusively.
> My only explanation of how this could happen is that dvp == *vpp and 
> cn_lkflags
> is LK_EXCLUSIVE.  In other words, this is a dot-dot lookup that results in the
> same vnode.  I think that this is only possible if dvp is the root vnode.
> I am not sure if my theory is correct though.
> Also, I am not sure if zfs_lookup() should be prepared to handle such a lookup
> or if this kind of lookup should be handled by upper/other layers.  In this 
> case
> these would be VFS lookup code and nullfs code.

So, is VV_ROOT flag set on the corresponding ZFS vnode ?

Just in case, you could try the following change, but I doubt that it
would have any effect.  Nullfs root vnode is cached so its VV_ROOT flag
should not be lost.  Also, I never seen similar issue with UFS.

diff --git a/sys/fs/nullfs/null_subr.c b/sys/fs/nullfs/null_subr.c
index fa6c4af..3f74579 100644
--- a/sys/fs/nullfs/null_subr.c
+++ b/sys/fs/nullfs/null_subr.c
@@ -251,6 +251,7 @@ null_nodeget(mp, lowervp, vpp)
        vp->v_type = lowervp->v_type;
        vp->v_data = xp;
        vp->v_vnlock = lowervp->v_vnlock;
+       vp->v_vflag = lowervp->v_vflag & VV_ROOT;
        error = insmntque1(vp, mp, null_insmntque_dtr, xp);
        if (error != 0)
                return (error);

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