Date:   Tue, 14 Feb 2023 11:48:07 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     Boqun Feng <boqun.feng@gmail.com>
Cc:     Alan Stern <stern@rowland.harvard.edu>,
        Kent Overstreet <kent.overstreet@linux.dev>,
        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>,
        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 05:51:11PM -0800, Boqun Feng wrote:
> On Mon, Feb 13, 2023 at 05:29:49PM +0100, Peter Zijlstra wrote:
> > On Mon, Feb 13, 2023 at 10:25:59AM -0500, Alan Stern wrote:
> > > 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.
> > > 
> > > Ah, this is news to me.  Is this sort of thing documented somewhere?
> 
> Basically if you have two lock instances A and B with the same class,
> and you know that locking ordering is always A -> B, then you can do
> 
> 	mutex_lock(A);
> 	mutex_lock_nest_lock(B, A); // lock B.
> 

No, this isn't quite right, You need at least 3 locks and 2 classes.

P, C1, C2

Then:

	mutex_lock(P)
	mutex_lock_next_lock(C1, P)
	mutex_lock_next_lock(C2, P)

And it will accept any order of Cn -- since it assumes that any
multi-lock of Cn will always hold P, therefore the ordering fully given
by P.