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Re: Layered fs, vnode locking and v_vnlock removal
On Tue, Jun 01, 2010 at 11:44:03AM +0200, Juergen Hannken-Illjes wrote:
> > It's not immediately clear how either of these ought to work, so
> > I'm concerned that making the infrastructure less general will
> > lead to problems.
> 1) One upper to many lower vnodes
> This is a file system like unionfs. It has to lock either one or
> many lower vnodes and does/will not earn anything of shared locks.
> 2) Many upper to one lower vnode
> Such a layered file system could use a lock shared between ALL
> upper and the lower vnode. Always taking the lower vnode's lock
> will do the same. I see no need for shared locks here.
That seems plausible.
> > In the long run I intend to make all the vnode ops symmetric with
> > respect to locking, which should make a lot of this less toxic, but at
> > the rate I've been able to work on this stuff we won't be there
> > anytime soon.
> The asymmetry comes from functions like null_mount() where a vnode gets
> locked by the lower layer and unlocked by the upper layer. A lower
> layer expecting its VOP_LOCK() to be matched by a VOP_UNLOCK() will
> fail badly.
...that is just broken, yes. If you can beat sanity into that, please do.
David A. Holland
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