linux-kernel - Re: [PATCH 1/2] pvclock: introduce seqcount-like API

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Date:	Wed, 6 Jul 2016 06:36:08 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Paolo Bonzini <pbonzini@...hat.com>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	kvm list <kvm@...r.kernel.org>, Ingo Molnar <mingo@...hat.com>,
	X86 ML <x86@...nel.org>, Minfei Huang <mnghuan@...il.com>
Subject: Re: [PATCH 1/2] pvclock: introduce seqcount-like API

I think you forget to pass -v3 to git format-patch :)

On Mon, Jun 27, 2016 at 7:19 AM, Paolo Bonzini <pbonzini@...hat.com> 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>
> ---
>         v2->v3: add unlikely() around read_retry check.
>
>  arch/x86/entry/vdso/vclock_gettime.c |  9 ++-------
>  arch/x86/include/asm/pvclock.h       | 39 +++++++++++++++++++++---------------
>  arch/x86/kernel/pvclock.c            | 17 ++++++----------
>  3 files changed, 31 insertions(+), 34 deletions(-)
>
> diff --git a/arch/x86/entry/vdso/vclock_gettime.c b/arch/x86/entry/vdso/vclock_gettime.c
> index 2f02d23a05ef..db1e3b4c3693 100644
> --- a/arch/x86/entry/vdso/vclock_gettime.c
> +++ b/arch/x86/entry/vdso/vclock_gettime.c
> @@ -123,9 +123,7 @@ static notrace cycle_t vread_pvclock(int *mode)
>          */
>
>         do {
> -               version = pvti->version;
> -
> -               smp_rmb();
> +               version = pvclock_read_begin(pvti);
>
>                 if (unlikely(!(pvti->flags & PVCLOCK_TSC_STABLE_BIT))) {
>                         *mode = VCLOCK_NONE;
> @@ -137,10 +135,7 @@ static notrace cycle_t vread_pvclock(int *mode)
>                 pvti_tsc_shift = pvti->tsc_shift;
>                 pvti_system_time = pvti->system_time;
>                 pvti_tsc = pvti->tsc_timestamp;
> -
> -               /* Make sure that the version double-check is last. */
> -               smp_rmb();
> -       } while (unlikely((version & 1) || version != pvti->version));
> +       } while (pvclock_read_retry(pvti, version));
>
>         delta = tsc - pvti_tsc;
>         ret = pvti_system_time +
> diff --git a/arch/x86/include/asm/pvclock.h b/arch/x86/include/asm/pvclock.h
> index 7c1c89598688..d019f0cc80ec 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)
> +{
> +       unsigned version = src->version & ~1;

I see your cute optimization here....

> +       /* 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)
> +{
> +       /* Make sure that the version is re-read after the data. */
> +       virt_rmb();
> +       return unlikely(version != src->version);

...and here.

LGTM.