linux-kernel - Re: [PATCH 08/19] VFS: introduce lookup_and

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Message-id: <173933579332.22054.11267140614769378076@noble.neil.brown.name>
Date: Wed, 12 Feb 2025 15:49:53 +1100
From: "NeilBrown" <neilb@...e.de>
To: "Al Viro" <viro@...iv.linux.org.uk>
Cc: "Christian Brauner" <brauner@...nel.org>, "Jan Kara" <jack@...e.cz>,
 "Linus Torvalds" <torvalds@...ux-foundation.org>,
 "Jeff Layton" <jlayton@...nel.org>, "Dave Chinner" <david@...morbit.com>,
 linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 08/19] VFS: introduce lookup_and_lock() and friends

On Sat, 08 Feb 2025, Al Viro wrote:
> On Thu, Feb 06, 2025 at 04:42:45PM +1100, NeilBrown wrote:
> > lookup_and_lock() combines locking the directory and performing a lookup
> > prior to a change to the directory.
> > Abstracting this prepares for changing the locking requirements.
> > 
> > done_lookup_and_lock() provides the inverse of putting the dentry and
> > unlocking.
> > 
> > For "silly_rename" we will need to lookup_and_lock() in a directory that
> > is already locked.  For this purpose we add LOOKUP_PARENT_LOCKED.
> 
> Ewww...  I do realize that such things might appear in intermediate
> stages of locking massage, but they'd better be _GONE_ by the end of it.
> Conditional locking of that sort is really asking for trouble.
> 
> If nothing else, better split the function in two variants and document
> the differences; that kind of stuff really does not belong in arguments.
> If you need it to exist through the series, that is - if not, you should
> just leave lookup_one_qstr() for the "locked" case from the very beginning.

That's what I did at first, but then when I realised I had to pass the
lookup flags around everywhere....  Will revert.

> 
> > This functionality is exported as lookup_and_lock_one() which takes a
> > name and len rather than a qstr.
> 
> ... for the sake of ...?

nfsd.

Thanks,
NeilBrown