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

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Date:	Thu, 9 Jun 2016 15:45:37 +0200
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Roman Kagan <rkagan@...tuozzo.com>, linux-kernel@...r.kernel.org,
	kvm@...r.kernel.org, mnghuan@...il.com, luto@...nel.org
Subject: Re: [PATCH] pvclock: introduce seqcount-like API



On 09/06/2016 15:35, Roman Kagan wrote:
> On Thu, Jun 09, 2016 at 02:47:54PM +0200, Paolo Bonzini wrote:
>> On 09/06/2016 14:43, Roman Kagan wrote:
>>> 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?
>>
>> Not this one, because Minfei's patch is enough to fix the bug, but I do
> 
> Has it landed in any public tree?  I'm unable to find any.  There
> appears to be another version of the patch on the list, so I'm confused.

I'm about to push it to kvm/master.

Paolo