linux-kernel - Re: [PATCH v5 6/7] locking/lockdep: Reuse freed chain

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Message-ID: <c3e01215-bb1c-e22a-32e3-f01e2e96584a@redhat.com>
Date:   Wed, 5 Feb 2020 09:03:28 -0500
From:   Waiman Long <longman@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Ingo Molnar <mingo@...hat.com>, Will Deacon <will.deacon@....com>,
        linux-kernel@...r.kernel.org, Bart Van Assche <bvanassche@....org>
Subject: Re: [PATCH v5 6/7] locking/lockdep: Reuse freed chain_hlocks entries

On 2/5/20 4:48 AM, Peter Zijlstra wrote:
> On Tue, Feb 04, 2020 at 11:57:09AM -0500, Waiman Long wrote:
>
>> Wait, it is possible that we can have deadlock like this:
>>
>>   cpu 0               cpu 1
>>   -----               -----
>>   lock A              lock B
>>   <irq>               <irq>
>>   lock B              lock A
>>  
>> If we eliminate 1-entry chain, will that impact our ability to detect this
>> kind of deadlock?
> I'm thinking that should trigger irq-inversion (irq-on vs in-irq) on
> either A or B (depending on timing).
>
> AFAICT the irq-state tracking is outside of validate_chain().
>
> This is also why I think your separate_irq_context() change is not
> needed.
>
> validate_chain() really only checks the per-context lock nesting, and
> there, a single lock doesn't do very much. Hence my proposed patch.
>
I see. Then it may be OK. I will take a further look just to be sure.

Cheers,
Longman