linux-kernel - Re: [PATCH] x86:pvclock: add missing barriers

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Date:	Wed, 8 Jun 2016 21:45:09 +0200
From:	Borislav Petkov <bp@...e.de>
To:	Roman Kagan <rkagan@...tuozzo.com>,
	Minfei Huang <mnghuan@...il.com>
Cc:	linux-kernel@...r.kernel.org, "Denis V. Lunev" <den@...nvz.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
	Andy Lutomirski <luto@...nel.org>,
	Paolo Bonzini <pbonzini@...hat.com>
Subject: Re: [PATCH] x86:pvclock: add missing barriers

On Wed, Jun 08, 2016 at 09:11:39PM +0300, Roman Kagan wrote:
> Gradual removal of excessive barriers in pvclock reading functions
> (commits 502dfeff239e8313bfbe906ca0a1a6827ac8481b,
> a3eb97bd80134ba07864ca00747466c02118aca1) ended up removing too much:
> although rdtsc is now orderd WRT other loads, there's no protection
> against the compiler reordering the loads of ->version with the loads of
> other fields.
> 
> E.g. on my system gcc-5.3.1 generates code which loads ->system_time and
> ->flags outside of the ->version test loop.
> 
> (Re)introduce the compiler barriers around accesses to the contents of
> pvclock.  While at this, make the function a bit more compact by
> removing unnecessary local variables.
> 
> Signed-off-by: Roman Kagan <rkagan@...tuozzo.com>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: Ingo Molnar <mingo@...hat.com>
> Cc: "H. Peter Anvin" <hpa@...or.com>
> Cc: x86@...nel.org
> Cc: Andy Lutomirski <luto@...nel.org>
> Cc: Borislav Petkov <bp@...e.de>
> Cc: Paolo Bonzini <pbonzini@...hat.com>
> Cc: stable@...r.kernel.org
> ---
>  arch/x86/include/asm/pvclock.h | 17 +++++------------
>  1 file changed, 5 insertions(+), 12 deletions(-)
> 
> diff --git a/arch/x86/include/asm/pvclock.h b/arch/x86/include/asm/pvclock.h
> index fdcc040..65c4de2 100644
> --- a/arch/x86/include/asm/pvclock.h
> +++ b/arch/x86/include/asm/pvclock.h
> @@ -80,18 +80,11 @@ static __always_inline
>  unsigned __pvclock_read_cycles(const struct pvclock_vcpu_time_info *src,
>  			       cycle_t *cycles, u8 *flags)
>  {
> -	unsigned version;
> -	cycle_t ret, offset;
> -	u8 ret_flags;
> -
> -	version = src->version;
> -
> -	offset = pvclock_get_nsec_offset(src);
> -	ret = src->system_time + offset;
> -	ret_flags = src->flags;
> -
> -	*cycles = ret;
> -	*flags = ret_flags;
> +	unsigned version = src->version;
> +	barrier();
> +	*cycles = src->system_time + pvclock_get_nsec_offset(src);
> +	*flags = src->flags;
> +	barrier();
>  	return version;

I have a similar patchset in my mbox starting here:

https://lkml.kernel.org/r/1464329832-4638-1-git-send-email-mnghuan@gmail.com

Care to take a look?

Thanks.


-- 
Regards/Gruss,
    Boris.

SUSE Linux GmbH, GF: Felix Imendörffer, Jane Smithard, Graham Norton, HRB 21284 (AG Nürnberg)
--