linux-kernel - Re: [patch 09/11] x86/vdso: Simplify the invalid vclock case

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Message-ID: <alpine.DEB.2.21.1809181515170.6950@nanos.tec.linutronix.de>
Date:   Tue, 18 Sep 2018 15:23:08 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Peter Zijlstra <peterz@...radead.org>
cc:     John Stultz <john.stultz@...aro.org>,
        Andy Lutomirski <luto@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>, X86 ML <x86@...nel.org>,
        Matt Rickard <matt@...trans.com.au>,
        Stephen Boyd <sboyd@...nel.org>,
        Florian Weimer <fweimer@...hat.com>,
        "K. Y. Srinivasan" <kys@...rosoft.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        devel@...uxdriverproject.org,
        Linux Virtualization <virtualization@...ts.linux-foundation.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Arnd Bergmann <arnd@...db.de>, Juergen Gross <jgross@...e.com>
Subject: Re: [patch 09/11] x86/vdso: Simplify the invalid vclock case

On Tue, 18 Sep 2018, Peter Zijlstra wrote:
> On Tue, Sep 18, 2018 at 12:41:57PM +0200, Thomas Gleixner wrote:
> > I still have one of the machines which is affected by this.
> 
> Are we sure this isn't a load vs rdtsc reorder? Because if I look at the
> current code:

The load order of last vs. rdtsc does not matter at all.

CPU0					CPU1

....
now0 = rdtsc_ordered();
...
tk->cycle_last = now0;

gtod->seq++;
gtod->cycle_last = tk->cycle_last;
...
gtod->seq++;
					seq_begin(gtod->seq);
					now1 = rdtsc_ordered();

So if the TSC on CPU1 is slightly behind the TSC on CPU0 then now1 can be
smaller than cycle_last. The TSC sync stuff does not catch the small delta
for unknown raisins. I'll go and find that machine and test that again.

Thanks,

	tglx