netdev - Re: [net-next 05/14] i40e: fix up 32 bit timespec references

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Message-ID: <CAK8P3a3fSofgDdLZ3S+MaEVz8DHnX6HO6xGiRWfCjQ614Kf4Cg@mail.gmail.com>
Date:   Mon, 25 Feb 2019 15:46:18 +0100
From:   Arnd Bergmann <arnd@...db.de>
To:     Jeff Kirsher <jeffrey.t.kirsher@...el.com>
Cc:     David Miller <davem@...emloft.net>,
        Jesse Brandeburg <jesse.brandeburg@...el.com>,
        Networking <netdev@...r.kernel.org>, nhorman@...hat.com,
        sassmann@...hat.com, jogreene@...hat.com
Subject: Re: [net-next 05/14] i40e: fix up 32 bit timespec references

On Wed, Jul 26, 2017 at 12:33 PM Jeff Kirsher
<jeffrey.t.kirsher@...el.com> wrote:
>
> From: Jesse Brandeburg <jesse.brandeburg@...el.com>
>
> As it turns out there was only a small set of errors
> on 32 bit, and we just needed to be using the right calls
> for dealing with timespec64 variables.

I just stumbled over code added by this older patch, and can't make sense
of the commit description here. Was this an attempt to fix a bug, or
just a cleanup?

>
> -       then = ns_to_timespec64(delta);
>         mutex_lock(&pf->tmreg_lock);
>
>         i40e_ptp_read(pf, &now);
> -       now = timespec64_add(now, then);
> +       timespec64_add_ns(&now, delta);
>         i40e_ptp_write(pf, (const struct timespec64 *)&now);

The problem I noticed here is that 'delta' is a user provided 64-bit
number from clock_adjtime(), and timespec64_add_ns() performs uses
a repeated addition instead of a div/mod pair. When the number
is large, we may end up adding a single second 8 billion times,
which may take a while even on a fast CPU.

Should the commit 0ac30ce43323 ("i40e: fix up 32 bit timespec
references") just be reverted?

      Arnd