linux-kernel - Re: [PATCH RFC] drivers/core: Replace lockdep_set_novalidate

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Date:   Mon, 13 Feb 2023 21:03:14 -0500
From:   Alan Stern <stern@...land.harvard.edu>
To:     Boqun Feng <boqun.feng@...il.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Kent Overstreet <kent.overstreet@...ux.dev>,
        Kent Overstreet <kent.overstreet@...il.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Coly Li <colyli@...e.de>,
        Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
        syzkaller <syzkaller@...glegroups.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Ingo Molnar <mingo@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>,
        USB list <linux-usb@...r.kernel.org>,
        Hillf Danton <hdanton@...a.com>
Subject: Re: [PATCH RFC] drivers/core: Replace lockdep_set_novalidate_class()
 with unique class keys

On Mon, Feb 13, 2023 at 05:51:11PM -0800, Boqun Feng wrote:
> 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);

Why isn't this treated as a deadlock?  It sure looks like a deadlock to 
me.  Is this an example where lockdep just doesn't get the right answer?

Alan Stern