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

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Message-ID: <abc4b4a6-d355-4dfd-a207-603e877b2b23@c-s.fr>
Date:   Tue, 24 Dec 2019 12:12:23 +0100
From:   christophe leroy <christophe.leroy@....fr>
To:     Andy Lutomirski <luto@...nel.org>
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>,
        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()



Le 24/12/2019 à 02:58, Andy Lutomirski a écrit :
> 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?

Euh ...

How can you return t when time pointer is NULL if you read t only when 
time pointer is not NULL ?

Christophe