Re: [PATCH 09/10] VFS: add LOOKUP_SILLY_RENAME

[NeilBrown]

On Sat, 27 Aug 2022, Al Viro wrote:
> On Fri, Aug 26, 2022 at 12:10:43PM +1000, NeilBrown wrote:
> > When performing a "silly rename" to avoid removing a file that is still
> > open, we need to perform a lookup in a directory that is already locked.
> > 
> > In order to allow common functions to be used for this lookup, introduce
> > LOOKUP_SILLY_RENAME which affirms that the directory is already locked
> > and that the vfsmnt is already writable.
> > 
> > When LOOKUP_SILLY_RENAME is set, path->mnt can be NULL.  As
> > i_op->rename() doesn't make the vfsmnt available, this is unavoidable.
> > So we ensure that a NULL ->mnt isn't fatal.
> 
> This one is really disgusting.  Flag-dependent locking is a pretty much
> guaranteed source of PITA and "magical" struct path is, again, asking for
> trouble.
> 
> You seem to be trying for simpler call graph and you end up paying with
> control flow that is much harder to reason about.
> 
It was mostly about avoiding code duplication.
I'll see if I can find a cleaner way.

Thanks,
NeilBrown