linux-kernel - Re: [PATCH 02/32] locking/lockdep: Introduce struct lock

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Date:   Tue, 12 Feb 2019 09:38:42 -0800
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Frederic Weisbecker <frederic@...nel.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Mauro Carvalho Chehab <mchehab@...pensource.com>,
        "David S . Miller" <davem@...emloft.net>,
        Thomas Gleixner <tglx@...utronix.de>,
        "Paul E . McKenney" <paulmck@...ux.vnet.ibm.com>,
        Frederic Weisbecker <fweisbec@...il.com>,
        Pavan Kondeti <pkondeti@...eaurora.org>,
        Ingo Molnar <mingo@...nel.org>,
        Joel Fernandes <joel@...lfernandes.org>
Subject: Re: [PATCH 02/32] locking/lockdep: Introduce struct lock_usage

On Tue, Feb 12, 2019 at 9:14 AM Frederic Weisbecker <frederic@...nel.org> wrote:
>
> +static u64 lock_usage_mask(struct lock_usage *usage)
> +{
> +       return BIT(usage->bit);
> +}

More insane "u64" - and it's *incorrect* too.

    #define BIT(nr)                    (1UL << (nr))

fundamentally means that "BIT()" can only work on up to "unsigned long".

So this odd use of u64 seems to be a disease. It only uses more memory
(and more CPU) for no obvious reason.

u64 is not some "default type". It's expensive and shouldn't be used
unless you have a *reason* for it.

              Linus