panic: knlist not locked, but should be

[Matthew Fleming]

> This appears to be an interaction with the recent changes to use 
> shared vnode locks for writes on ZFS.  Hmm, I think it may be ok to 
> use a shared vnode lock for kevents on vnodes though.  The vnode 
> interlock should be sufficient locking for what little work the kevent

> filters do.  As a quick hack for now the MNT_SHARED_WRITES() stuff 
> could avoid using shared locks 'if (!VN_KNLIST_EMPTY(vp))', but I 
> think the longer term fix is to not use the vnode locks for vnode
kevents, but use the interlock instead.

I tried (briefly) using the interlock since Isilon's vnode lock is
cluster wide (in our 6.1 based code we got away with using Giant).  This
got me a LOR report on the interlock:

	/*
	 * kqueue/VFS interaction
	 */
	{ "kqueue", &lock_class_mtx_sleep },
	{ "struct mount mtx", &lock_class_mtx_sleep },
	{ "vnode interlock", &lock_class_mtx_sleep },
	{ NULL, NULL },

since knote() will take first the list->kl_lock and then the kqueue
lock.  I didn't spend any time on it, and switched to using the vnode
v_lock for my purposes.  But someone added that lock ordering (r166421)
for a reason.

Cheers,
matthew