Date:   Mon, 13 Feb 2023 13:46:11 -0500
From:   Kent Overstreet <kent.overstreet@linux.dev>
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Alan Stern <stern@rowland.harvard.edu>,
        Kent Overstreet <kent.overstreet@gmail.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Coly Li <colyli@suse.de>,
        Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>,
        syzkaller <syzkaller@googlegroups.com>,
        Dmitry Vyukov <dvyukov@google.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        "Rafael J. Wysocki" <rafael@kernel.org>,
        Ingo Molnar <mingo@redhat.com>,
        Boqun Feng <boqun.feng@gmail.com>,
        LKML <linux-kernel@vger.kernel.org>,
        USB list <linux-usb@vger.kernel.org>,
        Hillf Danton <hdanton@sina.com>
Subject: Re: [PATCH RFC] drivers/core: Replace lockdep_set_novalidate_class()
 with unique class keys
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On Mon, Feb 13, 2023 at 10:24:13AM +0100, Peter Zijlstra wrote:
> On Sun, Feb 12, 2023 at 10:23:44AM -0500, Alan Stern wrote:
> > Provided it acquires the parent device's lock first, this is 
> > utterly safe no matter what order the children are locked in.  Try 
> > telling that to lockdep! 
> 
> mutex_lock_next_lock(child->lock, parent->lock) is there to express this
> exact pattern, it allows taking multiple child->lock class locks (in any
> order) provided parent->lock is held.

Perhaps I'm stupid, but I've never understood how subclasses - or this -
are supposed to work.

Locks don't get a fixed subclass, so what's to prevent some code from
going

/* thread 1: */
mutex_lock(&a->lock);
mutex_lock_nested(&b->lock, 1);

/* thread 2: */
mutex_lock(&b->lock);
mutex_lock_nested(&a->lock, 1);

I don't see how they can be used to check that we're obeying a lock
ordering?