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Date:   Mon, 2 Dec 2019 10:20:32 -0800
From:   "Luck, Tony" <tony.luck@intel.com>
To:     Sean Christopherson <sean.j.christopherson@intel.com>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@kernel.org>,
        Fenghua Yu <fenghua.yu@intel.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        H Peter Anvin <hpa@zytor.com>,
        Ashok Raj <ashok.raj@intel.com>,
        Ravi V Shankar <ravi.v.shankar@intel.com>,
        linux-kernel <linux-kernel@vger.kernel.org>, x86 <x86@kernel.org>
Subject: Re: [PATCH v10 6/6] x86/split_lock: Enable split lock detection by
 kernel parameter
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On Mon, Nov 25, 2019 at 08:13:48AM -0800, Sean Christopherson wrote:
> On Fri, Nov 22, 2019 at 04:30:56PM -0800, Luck, Tony wrote:
> > Don't you have some horrible races between the two logical
> > processors on the same core as they both try to set/clear the
> > MSR that is shared at the core level?
> 
> Yes and no.  Yes, there will be races, but they won't be fatal in any way.
> 
>   - Only the split-lock bit is supported by the kernel, so there isn't a
>     risk of corrupting other bits as both threads will rewrite the current
>     hardware value.
> 
>   - Toggling of split-lock is only done in "warn" mode.  Worst case
>     scenario of a race is that a misbehaving task will generate multiple
>     #AC exceptions on the same instruction.  And this race will only occur
>     if both siblings are running tasks that generate split-lock #ACs, e.g.
>     a race where sibling threads are writing different values will only
>     occur if CPUx is disabling split-lock after an #AC and CPUy is
>     re-enabling split-lock after *its* previous task generated an #AC.
> 
>   - Transitioning between modes at runtime isn't supported and disabling
>     is tracked per task, so hardware will always reach a steady state that
>     matches the configured mode.  I.e. split-lock is guaranteed to be
>     enabled in hardware once all _TIF_SLD threads have been scheduled out.

We should probably include this analysis in the commit
comment. Maybe a comment or two in the code too to note
that the races are mostly harmless and guaranteed to end
quickly.

-Tony