linux-kernel - Re: [PATCH] pvclock: introduce seqcount-like API

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Date:	Thu, 9 Jun 2016 15:43:17 +0300
From:	Roman Kagan <rkagan@...tuozzo.com>
To:	Paolo Bonzini <pbonzini@...hat.com>
CC:	<linux-kernel@...r.kernel.org>, <kvm@...r.kernel.org>,
	<mnghuan@...il.com>, <luto@...nel.org>
Subject: Re: [PATCH] pvclock: introduce seqcount-like API

On Thu, Jun 09, 2016 at 01:23:23PM +0200, Paolo Bonzini wrote:
> The version field in struct pvclock_vcpu_time_info basically implements
> a seqcount.  Wrap it with the usual read_begin and read_retry functions,
> and use these APIs instead of peppering the code with smp_rmb()s.
> While at it, change it to the more pedantically correct virt_rmb().
> 
> With this change, __pvclock_read_cycles can be simplified noticeably.
> 
> Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
> ---
>  arch/x86/entry/vdso/vclock_gettime.c | 25 +++++------------------
>  arch/x86/include/asm/pvclock.h       | 39 +++++++++++++++++++++---------------
>  arch/x86/kernel/pvclock.c            | 17 ++++++----------
>  3 files changed, 34 insertions(+), 47 deletions(-)
[...]
> --- a/arch/x86/include/asm/pvclock.h
> +++ b/arch/x86/include/asm/pvclock.h
[...]
> @@ -69,23 +87,12 @@ static inline u64 pvclock_scale_delta(u64 delta, u32 mul_frac, int shift)
>  }
>  
>  static __always_inline
> -unsigned __pvclock_read_cycles(const struct pvclock_vcpu_time_info *src,
> -			       cycle_t *cycles, u8 *flags)
> +cycle_t __pvclock_read_cycles(const struct pvclock_vcpu_time_info *src)
>  {
> -	unsigned version;
> -	cycle_t offset;
> -	u64 delta;
> -
> -	version = src->version;
> -	/* Make the latest version visible */
> -	smp_rmb();

This is on top of Minfei's patch, right?  It isn't in Linus' tree yet so
I wonder if it makes sense to merge the two patches into one.

Will you post it to stable, too?

Anyway

Reviewed-by: Roman Kagan <rkagan@...tuozzo.com>

Roman.