linux-kernel - Re: [PATCH 01/32] locking/lockdep: Use expanded masks on find_usage

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Message-ID: <CAHk-=wjYBy=TBCLwoh4v_7nf4kqLrt1h6D4CsVM7Ws2e06Efcg@mail.gmail.com>
Date:   Tue, 12 Feb 2019 09:35:34 -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 01/32] locking/lockdep: Use expanded masks on
 find_usage_*() functions

On Tue, Feb 12, 2019 at 9:14 AM Frederic Weisbecker <frederic@...nel.org> wrote:
>
> In order to perform softirq vector-finegrained locking validation we'll
> need to be able to check multiple vector usages at once. Prepare the low
> level usage mask check functions for that purpose.

Why is this using "u64 mask"?

That's not only fairly expensive on 32-bit targets, it wasn't what the
code did before:

> -static inline int usage_match(struct lock_list *entry, void *bit)
> +static inline int usage_match(struct lock_list *entry, void *mask)
>  {
> -       return entry->class->usage_mask & (1 << (enum lock_usage_bit)bit);
> +       return entry->class->usage_mask & *(u64 *)mask;

Note how that was an "int" mask value before, and "usage_mask" itself
is just "unsigned long".

So where does that "u64" come from?

If there is some reason you really want to use a 64-bit value (some
future change?), it should be documented.

                Linus