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

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Date: Mon, 5 Feb 2024 11:09:23 +0100
From: Jan Kara <jack@...e.cz>
To: Kent Overstreet <kent.overstreet@...ux.dev>
Cc: Jan Kara <jack@...e.cz>, 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 02-02-24 07:47:50, Kent Overstreet wrote:
> 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...

I see, thanks for explanation. So in the pipes case I actually like that
the patch makes the locking less obfuscated with lockdep details (to which
I'm mostly used to but still ;)). So feel free to add:

Reviewed-by: Jan Kara <jack@...e.cz>

for this patch. I'm not 100% convinced it will be always possible to
replace subclasses with the new ordering mechanism but I guess time will
show.

								Honza
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR