Feedback-ID: iad51458e:Fastmail
Date:   Mon, 13 Feb 2023 17:51:11 -0800
From:   Boqun Feng <boqun.feng@gmail.com>
To:     Peter Zijlstra <peterz@infradead.org>
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 05:29:49PM +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.

to tell the lockdep this is not deadlock, plus lockdep will treat the
acquisition of A and the precondition of acquisition B, so the following
is not a deadlock as well:

T1:
	mutex_lock(A);
	mutex_lock(C);
	mutex_lock_nest_lock(B, A);

T2:
	mutex_lock(A);
	mutex_lock_nest_lock(B, A);
	mutex_lock(C);

Regards,
Boqun

> 
> Probably not :/