linux-kernel - Re: [PATCH 24/28] locking/lockdep: Remove !dir in lock irq usage check

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Date:   Thu, 25 Apr 2019 22:03:36 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Yuyang Du <duyuyang@...il.com>
Cc:     will.deacon@....com, mingo@...nel.org, bvanassche@....org,
        ming.lei@...hat.com, frederic@...nel.org, tglx@...utronix.de,
        linux-kernel@...r.kernel.org, Boqun Feng <boqun.feng@...il.com>
Subject: Re: [PATCH 24/28] locking/lockdep: Remove !dir in lock irq usage
 check

On Wed, Apr 24, 2019 at 06:19:30PM +0800, Yuyang Du wrote:
> In mark_lock_irq(), the following checks are performed:
> 
>    ----------------------------------
>   |   ->      | unsafe | read unsafe |
>   |----------------------------------|
>   | safe      |  F  B  |    F* B*    |
>   |----------------------------------|
>   | read safe |  F? B* |      -      |
>    ----------------------------------
> 
> Where:
> F: check_usage_forwards
> B: check_usage_backwards
> *: check enabled by STRICT_READ_CHECKS
> ?: check enabled by the !dir condition
> 
> From checking point of view, the special F? case does not make sense,
> whereas it perhaps is made for peroformance concern. As later patch will
> address this issue, remove this exception, which makes the checks
> consistent later.
> 
> With STRICT_READ_CHECKS = 1 which is default, there is no functional
> change.

Oh man.. thinking required and it is way late.. anyway this whole read
stuff made me remember we had a patch set on readlocks last year.

  https://lkml.kernel.org/r/20180411135110.9217-1-boqun.feng@gmail.com

I remember reviewing that a few times and then it dropped on the floor,
probably because Spectre crap or something sucked up all my time again :/

Sorry Boqun!