Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
From:   Thomas Gleixner <tglx@linutronix.de>
To:     Christophe Leroy <christophe.leroy@c-s.fr>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Paul Mackerras <paulus@samba.org>,
        Michael Ellerman <mpe@ellerman.id.au>, arnd@arndb.de,
        vincenzo.frascino@arm.com, luto@kernel.org
Cc:     linux-kernel@vger.kernel.org, linuxppc-dev@lists.ozlabs.org,
        linux-arm-kernel@lists.infradead.org, linux-mips@vger.kernel.org,
        x86@kernel.org
Subject: Re: [RFC PATCH v2 07/10] lib: vdso: don't use READ_ONCE() in __c_kernel_time()
In-Reply-To: <fc1ff722c7cbe63a63ae02ade3a714d2049d54a5.1577111367.git.christophe.leroy@c-s.fr>
References: <cover.1577111363.git.christophe.leroy@c-s.fr> <fc1ff722c7cbe63a63ae02ade3a714d2049d54a5.1577111367.git.christophe.leroy@c-s.fr>
Date:   Fri, 10 Jan 2020 22:12:06 +0100
Message-ID: <87lfqfrp7d.fsf@nanos.tec.linutronix.de>
MIME-Version: 1.0
Content-Type: text/plain
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

Christophe Leroy <christophe.leroy@c-s.fr> writes:
>
> diff --git a/lib/vdso/gettimeofday.c b/lib/vdso/gettimeofday.c
> index 17b4cff6e5f0..5a17a9d2e6cd 100644
> --- a/lib/vdso/gettimeofday.c
> +++ b/lib/vdso/gettimeofday.c
> @@ -144,7 +144,7 @@ __cvdso_gettimeofday(const struct vdso_data *vd, struct __kernel_old_timeval *tv
>  static __maybe_unused __kernel_old_time_t
>  __cvdso_time(const struct vdso_data *vd, __kernel_old_time_t *time)
>  {
> -	__kernel_old_time_t t = READ_ONCE(vd[CS_HRES_COARSE].basetime[CLOCK_REALTIME].sec);
> +	__kernel_old_time_t t = vd[CS_HRES_COARSE].basetime[CLOCK_REALTIME].sec;
>  
>  	if (time)
>  		*time = t;

Allows the compiler to load twice, i.e. the returned value might be different from the
stored value. So no.

Thanks,

        tglx