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Re: Layered fs, vnode locking and v_vnlock removal
> On Wed, Jun 02, 2010 at 05:58:40PM +0100, David Laight wrote:
> > > > In the long term VOP_xxxLOCK() should become part of the file systems.
> > >
> > > AFAIK there is a consensus between yamt@, ad@ and thorpej@ that
> > > locking should be moved down to the filesystems.
> > > There was some discussion about it here some time before.
> Yes, this keeps coming up and I keep trying to explain why it's
and i keep failing to understand why it's misguided. :-)
> > There is a lurking problem making read/write atomically update the file
> > offset. I suspect that is currently covered by the vnode lock.
> > Might only affect O_APPEND - but I've seen systems get that wrong!
> > Not to mention the problem of correctly setting the file position
> > when read/write fault on a userspace address part way through a
> > transfer.
> Other important cases include atomicity of O_CREAT and permission
> checks done in VFS-level code.
> These cases can all be "handled" by cutting and pasting the code into
> every file system, but we really don't want to do that.
genfs-like approach is more appropriate than VOP_LOCK, imo.
> David A. Holland
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