linux-kernel - Re: [PATCH 02/32] locking/lockdep: lock_class_is

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Message-ID: <ZFqo50Gg3jFB1OEU@moria.home.lan>
Date:   Tue, 9 May 2023 16:11:19 -0400
From:   Kent Overstreet <kent.overstreet@...ux.dev>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-bcachefs@...r.kernel.org,
        Kent Overstreet <kent.overstreet@...il.com>,
        Ingo Molnar <mingo@...hat.com>, Will Deacon <will@...nel.org>,
        Waiman Long <longman@...hat.com>,
        Boqun Feng <boqun.feng@...il.com>
Subject: Re: [PATCH 02/32] locking/lockdep: lock_class_is_held()

On Tue, May 09, 2023 at 09:30:39PM +0200, Peter Zijlstra wrote:
> On Tue, May 09, 2023 at 12:56:27PM -0400, Kent Overstreet wrote:
> > From: Kent Overstreet <kent.overstreet@...il.com>
> > 
> > This patch adds lock_class_is_held(), which can be used to assert that a
> > particular type of lock is not held.
> 
> How is lock_is_held_type() not sufficient? Which is what's used to
> implement lockdep_assert_held*().

I should've looked at that before - it returns a tristate, so it's
closer than I thought, but this is used in contexts where we don't have
a lock or lockdep_map to pass and need to pass the lock_class_key
instead.

e.g, when initializing a btree_trans, or waiting on btree node IO, we
need to assert that no btree node locks are held.

Looking at the code, __lock_is_held() -> match_held_lock() has to care
about a bunch of stuff related to subclasses that doesn't seem relevant
to lock_class_is_held() - lock_class_is_held() is practically no code in
comparison, so I'm inclined to think they should just be separate.

But I'm not the lockdep expert :) Thoughts?