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

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Message-ID: <20250207202235.GH1977892@ZenIV>
Date: Fri, 7 Feb 2025 20:22:35 +0000
From: Al Viro <viro@...iv.linux.org.uk>
To: NeilBrown <neilb@...e.de>
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 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.

> This functionality is exported as lookup_and_lock_one() which takes a
> name and len rather than a qstr.

... for the sake of ...?