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

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Message-ID: <db49ab8a-a7be-4550-85b4-f4322ac9300a@redhat.com>
Date:	Thu, 9 Jun 2016 14:47:54 +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 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
plan on including it in 4.7 to simplify the merging (kvm/next has
already been branched off Linus's tree).

Paolo

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