Re: [PATCH v4] x86/nmi: Add an emergency handler in nmi_desc & use it in nmi_shootdown_cpus()

[Waiman Long <llong@redhat.com>]

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