netdev - Re: [PATCH 4/4] af_unix: convert to lock_cmp

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Date: Sun, 28 Jan 2024 14:38:02 -0500
From: Kent Overstreet <kent.overstreet@...ux.dev>
To: Kuniyuki Iwashima <kuniyu@...zon.com>
Cc: boqun.feng@...il.com, linux-fsdevel@...r.kernel.org, 
	linux-kernel@...r.kernel.org, netdev@...r.kernel.org, peterz@...radead.org
Subject: Re: [PATCH 4/4] af_unix: convert to lock_cmp_fn

On Sun, Jan 28, 2024 at 12:28:38AM -0800, Kuniyuki Iwashima wrote:
> From: Kent Overstreet <kent.overstreet@...ux.dev>
> Date: Fri, 26 Jan 2024 21:08:31 -0500
> > Kill
> >  - unix_state_lock_nested
> >  - _nested usage for net->unx.table.locks[].
> > 
> > replace both with lock_set_cmp_fn_ptr_order(&u->lock).
> > 
> > The lock ordering in sk_diag_dump_icons() looks suspicious; this may
> > turn up a real issue.
> 
> Yes, you cannot use lock_cmp_fn() for unix_state_lock_nested().
> 
> The lock order in sk_diag_dump_icons() is
> 
>   listening socket -> child socket in the listener's queue
> 
> , and the inverse order never happens.  ptr comparison does not make
> sense in this case, and lockdep will complain about false positive.

Is that a real lock ordering? Is this parent -> child relationship well
defined?

If it is, we should be able to write a lock_cmp_fn for it, as long as
it's not some handwavy "this will never happen but _nested won't check
for it" like I saw elsewhere in the net code... :)