linux-kernel - Re: [PATCH v2 3/4] x86/split_lock: Handle #AC exception for split lock

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Date:   Fri, 29 Jun 2018 10:29:08 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Fenghua Yu <fenghua.yu@...el.com>,
        "Luck, Tony" <tony.luck@...el.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, H Peter Anvin <hpa@...or.com>,
        "Raj, Ashok" <ashok.raj@...el.com>,
        Alan Cox <alan@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        "Wysocki, Rafael J" <rafael.j.wysocki@...el.com>,
        "Shankar, Ravi V" <ravi.v.shankar@...el.com>,
        linux-kernel <linux-kernel@...r.kernel.org>, x86 <x86@...nel.org>
Subject: Re: [PATCH v2 3/4] x86/split_lock: Handle #AC exception for split
 lock

On 06/29/2018 10:16 AM, Fenghua Yu wrote:
> On Fri, Jun 29, 2018 at 09:33:54AM -0700, Luck, Tony wrote:
>>>> +	WARN_ONCE(1, "A split lock issue is detected. Please FIX it\n");
>>>
>>> But, warning here is also not super useful.  Shouldn't we be dumping out
>>> the info in 'regs' instead of the current context?  We don't care about
>>> the state in the #AC handler, we care about 'regs'.
> 
> But WARN dump not only the state in the #AC handler, but also dump the regs
> in the current context. And WARN dumps stack.

Oh, I forgot about the fancy stack following.  That might give us useful
output, although mixed with useless output about the #AC handler.

But, in any case, could you please at least confirm that this does what
you think it does?  *Actually* generate #AC inside the kernel, with this
code, and share the output?

>> Maybe:
>>
>> 	WARN_ONCE(1, "split lock detected at %pF\n", regs[EIP]);
> 
> Should we dump redundant regs info while WARN shows them all already?

I bet it actually makes it easier to read the output and locate the real
source of the problem.  It's especially important if you're going to do
the WARN_() from the #AC handler with all the #AC information as noise
in the warning.