Harry Schmalzbauer wrote: 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. [stuff snipped for brevity] I took a look at this. (I know nothing about unionfs, but a little w.r.t. the VFS). I can confirm that this function (unionfs_nodeget()) is weird and appears to be broken to me.
The function calls insmntque() before it initializes the vnode, which seems
racey, especially if it isn't LK_EXCLUSIVE locked.
Also, line#s 278-281:
if (uppervp != NULLVP)
vp->v_vnlock = uppervp->v_vnlock;
else
vp->v_vnlock = lowervp->v_vnlock;
so your patch isn't locking the vnode lock that it actually uses.
I think the vp argument to insmntque() is required to be LK_EXCLUSIVE
locked mostly so other threads won't fiddle with the vnode until this
function is done with it, but I am not sure?
I think a more correct version of this (not saying it would be correct[😉],
would call insmntque() later in the function, after it has been initialized.
(This means that the cleanup if it fails is more involved, but...)
I've attached a patch (untested) that does this. Maybe you could try it?
rick
ps: I've cc'd Kostik, in case he has some insight w.r.t. how this should be
handled?
unionfs-newvnode.patch
Description: unionfs-newvnode.patch
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