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

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Date:	Sun, 12 Jun 2016 18:25:12 +0800
From:	Minfei Huang <mnghuan@...il.com>
To:	Paolo Bonzini <pbonzini@...hat.com>
Cc:	linux-kernel@...r.kernel.org, kvm@...r.kernel.org, luto@...nel.org,
	rkagan@...tuozzo.com
Subject: Re: [PATCH] pvclock: introduce seqcount-like API

On 06/09/16 at 01:23P, 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.

Hi, Paolo.

Thanks for accepting my patches in your repo.

> 
> 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(-)
> 
> diff --git a/arch/x86/include/asm/pvclock.h b/arch/x86/include/asm/pvclock.h
> index 7c1c89598688..0ee92db1e9f3 100644
> --- a/arch/x86/include/asm/pvclock.h
> +++ b/arch/x86/include/asm/pvclock.h
> @@ -25,6 +25,24 @@ void pvclock_resume(void);
>  
>  void pvclock_touch_watchdogs(void);
>  
> +static __always_inline
> +unsigned pvclock_read_begin(const struct pvclock_vcpu_time_info *src)

It's better to use type unsigned int, instead of unsigned which is
complained by script checkpatch.

Thanks
Minfei

> +{
> +	unsigned version = src->version & ~1;

Ditto.

> +	/* Make sure that the version is read before the data. */
> +	virt_rmb();
> +	return version;
> +}
> +
> +static __always_inline
> +bool pvclock_read_retry(const struct pvclock_vcpu_time_info *src,
> +			unsigned version)

Ditto.

Thanks
Minfei