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

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Message-ID: <20200205094838.GI14879@hirez.programming.kicks-ass.net>
Date:   Wed, 5 Feb 2020 10:48:38 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Waiman Long <longman@...hat.com>
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 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.