linux-kernel - Re: [RFC PATCH 00/16] x86/split_lock: Enable #AC exception for split locked accesses

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Message-ID: <alpine.DEB.2.21.1806221118250.2402@nanos.tec.linutronix.de>
Date:   Fri, 22 Jun 2018 11:22:09 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Fenghua Yu <fenghua.yu@...el.com>
cc:     Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...e.hu>,
        "H. Peter Anvin" <hpa@...ux.intel.com>,
        Ashok Raj <ashok.raj@...el.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Rafael Wysocki <rafael.j.wysocki@...el.com>,
        Tony Luck <tony.luck@...el.com>,
        Alan Cox <alan@...ux.intel.com>,
        Ravi V Shankar <ravi.v.shankar@...el.com>,
        Arjan van de Ven <arjan@...radead.org>,
        linux-kernel <linux-kernel@...r.kernel.org>, x86 <x86@...nel.org>
Subject: Re: [RFC PATCH 00/16] x86/split_lock: Enable #AC exception for split
 locked accesses

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