Re: [PATCH 1/4] fs/pipe: Convert to lockdep_cmp_fn

From: Jan Kara <jack@suse.cz>
To: Sedat Dilek <sedat.dilek@gmail.com>
Cc: Jan Kara <jack@suse.cz>,
	Kent Overstreet <kent.overstreet@linux.dev>,
	linux-kernel@vger.kernel.org, netdev@vger.kernel.org,
	linux-fsdevel@vger.kernel.org, peterz@infradead.org,
	boqun.feng@gmail.com, Alexander Viro <viro@zeniv.linux.org.uk>,
	Christian Brauner <brauner@kernel.org>
Subject: Re: [PATCH 1/4] fs/pipe: Convert to lockdep_cmp_fn
Date: Mon, 5 Feb 2024 10:53:58 +0100	[thread overview]
Message-ID: <20240205095358.p322zf5u74fgczlo@quack3> (raw)
In-Reply-To: <CA+icZUWdUUhWg_-0NvF+6L=EUhj7amv_7HRKHDPvrEBspwHC2Q@mail.gmail.com>

On Fri 02-02-24 13:25:20, Sedat Dilek wrote:
> On Fri, Feb 2, 2024 at 1:12 PM Jan Kara <jack@suse.cz> 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@zeniv.linux.org.uk>
> > > Cc: Christian Brauner <brauner@kernel.org>
> > > Cc: Jan Kara <jack@suse.cz>
> > > Signed-off-by: Kent Overstreet <kent.overstreet@linux.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?
> >
> >                                                                 Honza
> 
> Hi,
> 
> "Numbers talk - Bullshit walks." (Linus Torvalds)
> 
> In things of pipes - I normally benchmark like this (example):
> 
> root# cat /dev/sdc | pipebench > /dev/null
> 
> Do you have numbers for your patch-series?

Sedat AFAIU this patch is not about performance at all but rather about
lockdep instrumentation... But maybe I'm missing your point?

								Honza

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