linux-kernel - Re: [PATCH] x86/vdso/pvclock: Protect STABLE check with the seqcount

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Date:	Thu, 7 Jan 2016 22:47:47 +0100
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Andy Lutomirski <luto@...capital.net>,
	Marcelo Tosatti <mtosatti@...hat.com>
Cc:	Andy Lutomirski <luto@...nel.org>, X86 ML <x86@...nel.org>,
	Radim Krcmar <rkrcmar@...hat.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	kvm list <kvm@...r.kernel.org>, Alexander Graf <agraf@...e.de>
Subject: Re: [PATCH] x86/vdso/pvclock: Protect STABLE check with the seqcount



On 07/01/2016 22:13, Andy Lutomirski wrote:
> I don't understand what you mean.
> 
> In the old code (4.3 and 4.4), the vdso checks STABLE_BIT at the end,
> which is correct as long as STABLE_BIT can never change from 0 to 1.
> 
> In the -tip code, it's clearly wrong.
> 
> In the code in this patch, it should be correct regardless of how
> STABLE_BIT changes as long as the seqcount works.  Given that the
> performance cost of doing that is zero, I'd rather keep it that way.
> If we're really paranoid, we could move it after the rest of the pvti
> reads and add a barrier, but is there really any host on which that
> matters?

I agree that your patch is fine.

Reviewed-by: Paolo Bonzini <pbonzini@...hat.com>

Paolo