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

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Date:   Fri, 10 Jan 2020 22:12:06 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Christophe Leroy <christophe.leroy@....fr>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>, arnd@...db.de,
        vincenzo.frascino@....com, luto@...nel.org
Cc:     linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        linux-arm-kernel@...ts.infradead.org, linux-mips@...r.kernel.org,
        x86@...nel.org
Subject: Re: [RFC PATCH v2 07/10] lib: vdso: don't use READ_ONCE() in __c_kernel_time()

Christophe Leroy <christophe.leroy@....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