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Re: Layered fs, vnode locking and v_vnlock removal
On Tue, Jun 01, 2010 at 01:54:53AM +0000, David Holland wrote:
> On Sun, May 23, 2010 at 04:11:25PM +0200, Juergen Hannken-Illjes wrote:
> > With our current vnode lock implementation VOP_LOCK() and VOP_UNLOCK()
> > are not symmetric. A vnode may be locked from one file system and
> > unlocked from another one. Is there any reason left to have layered
> > file systems share the vnode lock with lower file systems via v_vnlock?
> > The attached patch will remove v_vnlock and change layered file systems
> > to always pass the locking VOP's down to the leaf file system.
> I'm not sure. This is fine for things like umapfs and nullfs where
> there's a 1-1 mapping between upper and lower vnodes; but what if
> there isn't? Consider for example a versioning FS layer that stores
> versions as diffs, or a layer that turns /var/mail into maildirs. 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
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.
> 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
In the long term VOP_xxxLOCK() should become part of the file systems.
> David A. Holland
Juergen Hannken-Illjes - hannken%eis.cs.tu-bs.de@localhost - TU Braunschweig
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