Feedback-ID: iad51458e:Fastmail
Date:   Mon, 13 Feb 2023 21:55:06 -0800
From:   Boqun Feng <boqun.feng@gmail.com>
To:     Hillf Danton <hdanton@sina.com>
Cc:     Alan Stern <stern@rowland.harvard.edu>,
        Peter Zijlstra <peterz@infradead.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        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>
Subject: Re: [PATCH RFC] drivers/core: Replace lockdep_set_novalidate_class()
 with unique class keys
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On Tue, Feb 14, 2023 at 01:27:33PM +0800, Hillf Danton wrote:
> On Mon, 13 Feb 2023 18:09:16 -0800 Boqun Feng <boqun.feng@gmail.com>
> > On Mon, Feb 13, 2023 at 09:03:14PM -0500, Alan Stern wrote:
> > > 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?
> 
> Syzbot reported deadlock[1] with A ignored.
> 
> [1] https://lore.kernel.org/linux-mm/20230130073136.59-1-hdanton@sina.com/
> 

Right, I think that's a false positive, however it's not related to
mutex_lock_nest_lock(). Anyway mutex_lock_nest_lock() cannot help that
case since these are three different lock class.

Actually, reading the code again, I think I made a mistake, for
mutex_lock_nest_lock(), the following *is* a deadlock to lockdep:

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);

and this *is not* a deadlock to lockdep:

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

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

The current semantics of _nest_lock() is tricky, it only provides the
"nest" effect if it is the next lock acquired after the "parent" lock.
Maybe we can change and make it clear a little bit to make it more
useful.

Ah, actually someone found it 7 years ago:

https://lore.kernel.org/lkml/20150810095247.GA4606@fixme-laptop.cn.ibm.com/

;-)

Regards,
Boqun

> > Because A serializes B and C, so that particular piece of code doesn't
> > cause deadlock. Note that you can still use you normal mutex_lock() for
> > B, so if there is more code:
> > 
> > T3:
> > 	mutex_lock(C);
> > 	mutex_lock(B);
> > 
> > lockdep will report deadlock.
> > 
> > Regards,
> > Boqun
> > 
> > > Alan Stern