linux-kernel - Re: [PATCH 3/3 V2] pvclock: Get rid of __pvclock_read_cycles in function pvclock_read

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Date:	Wed, 8 Jun 2016 10:17:39 +0200
From:	Borislav Petkov <bp@...e.de>
To:	Minfei Huang <mnghuan@...il.com>
Cc:	pbonzini@...hat.com, luto@...nel.org, hpa@...or.com,
	mingo@...hat.com, tglx@...utronix.de, x86@...nel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/3 V2] pvclock: Get rid of __pvclock_read_cycles in
 function pvclock_read_flags

On Sat, May 28, 2016 at 08:27:43PM +0800, Minfei Huang wrote:
> There is a generic function __pvclock_read_cycles to be used to get both
> flags and cycles. For function pvclock_read_flags, it's useless to get
> cycles value. To make this function be more effective, get this variable
> flags directly in function.
> 
> Signed-off-by: Minfei Huang <mnghuan@...il.com>
> ---
> v1:
> - Get rid of __pvclock_read_cycles according to Andy's suggestion
> ---
>  arch/x86/kernel/pvclock.c | 7 +++++--
>  1 file changed, 5 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/kernel/pvclock.c b/arch/x86/kernel/pvclock.c
> index 7f82fe0..06c58ce 100644
> --- a/arch/x86/kernel/pvclock.c
> +++ b/arch/x86/kernel/pvclock.c
> @@ -61,11 +61,14 @@ void pvclock_resume(void)
>  u8 pvclock_read_flags(struct pvclock_vcpu_time_info *src)
>  {
>  	unsigned version;
> -	cycle_t ret;
>  	u8 flags;
>  
>  	do {
> -		version = __pvclock_read_cycles(src, &ret, &flags);
> +		version = src->version;
> +		/* Make the latest version visible */
> +		smp_rmb();
> +
> +		flags = src->flags;
>  		/* Make sure that the version double-check is last. */

What does that comment mean over the barrier? It should be over the
"while" line IMO.

>  		smp_rmb();

Why the two barriers back-to-back? Can't have one at the end for all?

>  	} while ((src->version & 1) || version != src->version);

-- 
Regards/Gruss,
    Boris.

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