linux-kernel - Re: [PATCH 1/4] fs/pipe: Convert to lockdep_cmp

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Date: Fri, 2 Feb 2024 07:47:50 -0500
From: Kent Overstreet <kent.overstreet@...ux.dev>
To: Jan Kara <jack@...e.cz>
Cc: linux-kernel@...r.kernel.org, netdev@...r.kernel.org, 
	linux-fsdevel@...r.kernel.org, peterz@...radead.org, boqun.feng@...il.com, 
	Alexander Viro <viro@...iv.linux.org.uk>, Christian Brauner <brauner@...nel.org>
Subject: Re: [PATCH 1/4] fs/pipe: Convert to lockdep_cmp_fn

On Fri, Feb 02, 2024 at 01:03:57PM +0100, Jan Kara wrote:
> On Fri 26-01-24 21:08:28, Kent Overstreet wrote:
> > *_lock_nested() is fundamentally broken; lockdep needs to check lock
> > ordering, but we cannot device a total ordering on an unbounded number
> > of elements with only a few subclasses.
> > 
> > the replacement is to define lock ordering with a proper comparison
> > function.
> > 
> > fs/pipe.c was already doing everything correctly otherwise, nothing
> > much changes here.
> > 
> > Cc: Alexander Viro <viro@...iv.linux.org.uk>
> > Cc: Christian Brauner <brauner@...nel.org>
> > Cc: Jan Kara <jack@...e.cz>
> > Signed-off-by: Kent Overstreet <kent.overstreet@...ux.dev>
> 
> I had to digest for a while what this new lockdep lock ordering feature is
> about. I have one pending question - what is the motivation of this
> conversion of pipe code? AFAIU we don't have any problems with lockdep
> annotations on pipe->mutex because there are always only two subclasses?

It's one of the easier conversions to do, and ideally /all/ users of
subclasses would go away.

Start with the easier ones, figure out those patterns, then the
harder...