linux-kernel - Re: [PATCH 05/32] locking/lockdep: Prepare valid

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Date:   Tue, 12 Feb 2019 09:45:52 -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 05/32] locking/lockdep: Prepare valid_state() to handle
 plain masks

On Tue, Feb 12, 2019 at 9:14 AM Frederic Weisbecker <frederic@...nel.org> wrote:
>
> +
> +       while (vectors) {
> +               long fs = __ffs64(vectors) + 1;
> +
> +               vectors >>= fs;

This is wrong.

If "vectors" only has the high hit set, you end up with "fs" having
the value "64".

And then "vectors >>= fs" is undefined and won't actually do anything
at all on x86.

In general, avoid "ffs()", and the stupid pattern of "__ffs(x)+1".

Bit numbering starts at 0. "ffs()" is wrong. And you never *ever* just
add one to a bit number in order to shift by one more bit, exactly
because of overflow issues.

So it may look inefficient, but the correct thing to do is

    long bit = __ffs64(vectors);
    vectors >>= fs;
    vectors >>= 1;

because that actually works.

                     Linus