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Date:   Wed, 28 Oct 2020 20:42:08 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     Chris Wilson <chris@chris-wilson.co.uk>
Cc:     linux-kernel@vger.kernel.org, linux-tip-commits@vger.kernel.org,
        tip-bot2 for Peter Zijlstra <tip-bot2@linutronix.de>,
        Qian Cai <cai@redhat.com>, x86 <x86@kernel.org>,
        Boqun Feng <boqun.feng@gmail.com>
Subject: Re: [tip: locking/core] lockdep: Fix usage_traceoverflow
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On Wed, Oct 28, 2020 at 05:40:48PM +0000, Chris Wilson wrote:
> Quoting Chris Wilson (2020-10-27 16:34:53)
> > Quoting Peter Zijlstra (2020-10-27 15:45:33)
> > > On Tue, Oct 27, 2020 at 01:29:10PM +0000, Chris Wilson wrote:
> > > 
> > > > <4> [304.908891] hm#2, depth: 6 [6], 3425cfea6ff31f7f != 547d92e9ec2ab9af
> > > > <4> [304.908897] WARNING: CPU: 0 PID: 5658 at kernel/locking/lockdep.c:3679 check_chain_key+0x1a4/0x1f0
> > > 
> > > Urgh, I don't think I've _ever_ seen that warning trigger.
> > > 
> > > The comments that go with it suggest memory corruption is the most
> > > likely trigger of it. Is it easy to trigger?
> > 
> > For the automated CI, yes, the few machines that run that particular HW
> > test seem to hit it regularly. I have not yet reproduced it for myself.
> > I thought it looked like something kasan would provide some insight for
> > and we should get a kasan run through CI over the w/e. I suspect we've
> > feed in some garbage and called it a lock.
> 
> I tracked it down to a second invocation of lock_acquire_shared_recursive()
> intermingled with some other regular mutexes (in this case ww_mutex).
> 
> We hit this path in validate_chain():
> 	/*
> 	 * Mark recursive read, as we jump over it when
> 	 * building dependencies (just like we jump over
> 	 * trylock entries):
> 	 */
> 	if (ret == 2)
> 		hlock->read = 2;
> 
> and that is modifying hlock_id() and so the chain-key, after it has
> already been computed.

Ooh, interesting.. I'll have to go look at this in the morning, brain is
fried already. Thanks for digging into it.