linux-kernel - Re: [PATCH v4] x86/nmi: Add an emergency handler in nmi_desc & use it in nmi_shootdown

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Message-ID: <2ed01b76-0a8d-48ec-a366-40bca78de93e@redhat.com>
Date: Thu, 6 Feb 2025 12:00:31 -0500
From: Waiman Long <llong@...hat.com>
To: Thomas Gleixner <tglx@...utronix.de>, Waiman Long <llong@...hat.com>,
 Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
 Dave Hansen <dave.hansen@...ux.intel.com>,
 Peter Zijlstra <peterz@...radead.org>
Cc: x86@...nel.org, linux-kernel@...r.kernel.org,
 "H. Peter Anvin" <hpa@...or.com>, Rik van Riel <riel@...riel.com>
Subject: Re: [PATCH v4] x86/nmi: Add an emergency handler in nmi_desc & use it
 in nmi_shootdown_cpus()

On 2/6/25 11:14 AM, Thomas Gleixner wrote:
> On Wed, Feb 05 2025 at 21:46, Waiman Long wrote:
>> On 2/5/25 4:20 AM, Thomas Gleixner wrote:
>>>> +int set_emergency_nmi_handler(unsigned int type, nmi_handler_t handler)
>>>> +{
>>>> +	struct nmi_desc *desc = nmi_to_desc(type);
>>>> +	nmi_handler_t orig = NULL;
>>>> +
>>>> +	if (!handler) {
>>>> +		orig = READ_ONCE(desc->emerg_handler);
>>>> +		WARN_ON_ONCE(!orig);
>>>> +	}
>>>> +
>>>> +	if (try_cmpxchg(&desc->emerg_handler, &orig, handler))
>>>> +		return 0;
>>> What's the point of this cmpxchg()? What's the concurrency problem this
>>> tries to address?
>> It is because I am not sure if there can only be one instance of
>> nmi_shootdown_cpus() at any time. If there can't be more than one
>> instance, I can remove the atomic instruction.
> There are two ways to get there:
>
>    1) crash(), which installs a handler
>
>    2) emergency_reboot(), which sets the handler to NULL
>
> If they interfere, then the callback is the least of your
> worries. That's already broken today in so many other ways.

OK, I will remove the atomic instruction in the next version. Thanks for 
the explanation.

Cheers,
Longman