linux-kernel - Re: [PATCH 3/3 V2] pvclock: Get rid of __pvclock_read_cycles in function pvclock_read

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Date:	Thu, 9 Jun 2016 14:26:59 +0200
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Peter Zijlstra <peterz@...radead.org>,
	Minfei Huang <mnghuan@...il.com>
Cc:	bp@...e.de, luto@...nel.org, hpa@...or.com, mingo@...hat.com,
	tglx@...utronix.de, x86@...nel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/3 V2] pvclock: Get rid of __pvclock_read_cycles in
 function pvclock_read_flags



On 09/06/2016 14:16, Peter Zijlstra wrote:
> On Sat, May 28, 2016 at 08:27:43PM +0800, Minfei Huang wrote:
>> +++ b/arch/x86/kernel/pvclock.c
>> @@ -61,11 +61,14 @@ void pvclock_resume(void)
>>  u8 pvclock_read_flags(struct pvclock_vcpu_time_info *src)
>>  {
>>  	unsigned version;
>> -	cycle_t ret;
>>  	u8 flags;
>>  
>>  	do {
>> -		version = __pvclock_read_cycles(src, &ret, &flags);
>> +		version = src->version;
>> +		/* Make the latest version visible */
>> +		smp_rmb();
>> +
>> +		flags = src->flags;
> 
> Using a seqcount to load a single byte is insane ;-)

Only if you know that the writer will not write that byte twice within a
critical section...

Which I guess we do know in this case because the write side is just a
memcpy, but it's still a bit safer when it's not specified by the
pvclock API.  It's not a fast path anyway, it runs literally twice at
startup.

Paolo