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Message-ID: <Y+rtTLD9UtPI5uGM@boqun-archlinux>
Date: Mon, 13 Feb 2023 18:09:16 -0800
From: Boqun Feng <boqun.feng@...il.com>
To: Alan Stern <stern@...land.harvard.edu>
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 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?
>
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
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