Re: Layered fs, vnode locking and v_vnlock removal

[yamt%mwd.biglobe.ne.jp@localhost (YAMAMOTO Takashi)]

hi,

> 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
> misguided.

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.

YAMAMOTO Takashi

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