Re: Move the vnode lock into file systems

[David Holland <dholland-tech%netbsd.org@localhost>]

On Sat, Jun 26, 2010 at 10:39:27AM +0200, Juergen Hannken-Illjes wrote:
 > The vnode lock operations currently work on a rw lock located inside the
 > vnode.  I propose to move this lock into the file system node.
 > 
 > This place is more logical as we lock a file system node and not a vnode.
 > This becomes clear if we think of a file system where one file system node
 > is attached to more than one vnode.  Ptyfs allowing multiple mounts is such
 > a candidate.

I'm not convinced that sharing locks at the VFS level is a good idea
for such cases. While ptyfs specifically is pretty limited and
unlikely to get into trouble, something more complex easily could. I
don't think we ought to encourage this until we have a clear plan for
rebind mounts and the resulting namespace-handling issues. (Since
in a sense that's the general case of multiply-mounted ptyfs.)

Since I'm pretty sure that a reasonable architecture for that will
lead to sharing vnodes between the multiple mountpoints, and
manifesting some kind of virtual name objects akin to Linux's dentries
to keep track of which mountpoint is which, I don't see that it's
necessary or desirable to specialize the locking...

Do you have another use case in mind?

(In the absence of some clear benefits I don't think it's a
particularly good idea to paste a dozen or two copies of genfs_lock
everywhere. But folding vcrackmgr() into genfs_lock and genfs_unlock
seems like a fine idea.)

-- 
David A. Holland
dholland%netbsd.org@localhost