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

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Date:   Fri, 26 Apr 2019 15:06:34 +0800
From:   Yuyang Du <duyuyang@...il.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     will.deacon@....com, Ingo Molnar <mingo@...nel.org>,
        Bart Van Assche <bvanassche@....org>, ming.lei@...hat.com,
        Frederic Weisbecker <frederic@...nel.org>, tglx@...utronix.de,
        LKML <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

Thanks for review.

On Fri, 26 Apr 2019 at 04:03, Peter Zijlstra <peterz@...radead.org> wrote:
>
> 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!

Oh man, I thought about the read-write lock stuff, but I didn't know
Boqun's patch. Let me hurt my brain looking at that patch.