linux-kernel - Re: [PATCH 09/10] VFS: add LOOKUP_SILLY

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Message-id: <166174295243.27490.1036858614514220411@noble.neil.brown.name>
Date:   Mon, 29 Aug 2022 13:15:52 +1000
From:   "NeilBrown" <neilb@...e.de>
To:     "Al Viro" <viro@...iv.linux.org.uk>
Cc:     "Linus Torvalds" <torvalds@...ux-foundation.org>,
        "Daire Byrne" <daire@...g.com>,
        "Trond Myklebust" <trond.myklebust@...merspace.com>,
        "Chuck Lever" <chuck.lever@...cle.com>,
        "Linux NFS Mailing List" <linux-nfs@...r.kernel.org>,
        linux-fsdevel@...r.kernel.org,
        "LKML" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 09/10] VFS: add LOOKUP_SILLY_RENAME

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