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Date:   Fri, 22 Jun 2018 11:22:09 +0200 (CEST)
From:   Thomas Gleixner <tglx@linutronix.de>
To:     Fenghua Yu <fenghua.yu@intel.com>
cc:     Peter Zijlstra <peterz@infradead.org>, Ingo Molnar <mingo@elte.hu>,
        "H. Peter Anvin" <hpa@linux.intel.com>,
        Ashok Raj <ashok.raj@intel.com>,
        Dave Hansen <dave.hansen@intel.com>,
        Rafael Wysocki <rafael.j.wysocki@intel.com>,
        Tony Luck <tony.luck@intel.com>,
        Alan Cox <alan@linux.intel.com>,
        Ravi V Shankar <ravi.v.shankar@intel.com>,
        Arjan van de Ven <arjan@infradead.org>,
        linux-kernel <linux-kernel@vger.kernel.org>, x86 <x86@kernel.org>
Subject: Re: [RFC PATCH 00/16] x86/split_lock: Enable #AC exception for split
 locked accesses
In-Reply-To: <20180621220003.GD114883@romley-ivt3.sc.intel.com>
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References: <1527435965-202085-1-git-send-email-fenghua.yu@intel.com> <20180621193738.GA13636@worktop.programming.kicks-ass.net> <20180621201851.GC114883@romley-ivt3.sc.intel.com> <alpine.DEB.2.21.1806212223480.1591@nanos.tec.linutronix.de>
 <20180621220003.GD114883@romley-ivt3.sc.intel.com>
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On Thu, 21 Jun 2018, Fenghua Yu wrote:
> The control knobs allow sysadmin to handle #AC for split lock in different
> scenarios and usages.
> 
> The control knob for kernel is to choose re-executing the faulting
> instruction (default) or kernel panic. Kernel panic may be useful in hard
> real time which has less tolerant to bad performance.

That's nonsense, really.

 1) The re-executing mechanism is broken and totally useless

 2) Panicing a real-time system just due to a single #AC is total
    overkill. Real-Time systems care very much about proper safe state
    transitioning. Panic is surely a safe state, but so is power off. But
    neither of them qualifies as proper state transitioning.

Thanks,

	tglx