linux-kernel - Re: [RFC PATCH v2 07/10] lib: vdso: don't use READ_ONCE() in __c_kernel

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Message-ID: <CALCETrV1TWD4iMoGFX6abxXFukz081Y2XaXy6p1KJBNLpp6ZTA@mail.gmail.com>
Date:   Mon, 23 Dec 2019 17:58:24 -0800
From:   Andy Lutomirski <luto@...nel.org>
To:     Christophe Leroy <christophe.leroy@....fr>
Cc:     Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Arnd Bergmann <arnd@...db.de>,
        Thomas Gleixner <tglx@...utronix.de>,
        Vincenzo Frascino <vincenzo.frascino@....com>,
        Andrew Lutomirski <luto@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        "open list:MIPS" <linux-mips@...r.kernel.org>,
        X86 ML <x86@...nel.org>
Subject: Re: [RFC PATCH v2 07/10] lib: vdso: don't use READ_ONCE() in __c_kernel_time()

On Mon, Dec 23, 2019 at 6:31 AM Christophe Leroy
<christophe.leroy@....fr> wrote:
>
> READ_ONCE() forces the read of the 64 bit value of
> vd[CS_HRES_COARSE].basetime[CLOCK_REALTIME].sec allthough
> only the lower part is needed.

Seems reasonable and very unlikely to be harmful.  That being said,
this function really ought to be considered deprecated -- 32-bit
time_t is insufficient.

Do you get even better code if you move the read into the if statement?

Reviewed-by: Andy Lutomirski <luto@...nel.org>

--Andy