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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

 > 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.

David A. Holland

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