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 13:21:18 +0200
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Borislav Petkov <bp@...e.de>, Minfei Huang <mnghuan@...il.com>
Cc:	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 08/06/2016 10:17, Borislav Petkov wrote:
>> > -		version = __pvclock_read_cycles(src, &ret, &flags);
>> > +		version = src->version;
>> > +		/* Make the latest version visible */
>> > +		smp_rmb();
>> > +
>> > +		flags = src->flags;
>> >  		/* Make sure that the version double-check is last. */
> What does that comment mean over the barrier? It should be over the
> "while" line IMO.
> 
>> >  		smp_rmb();
> Why the two barriers back-to-back? Can't have one at the end for all?
> 

This is basically implementing a seqcount.  It needs two barriers and,
technically, they should be virt_rmb() -- it really doesn't matter of
course because reads are never reordered on x86.

Paolo