Re: [PATCH] x86/nmi: Use trylock in __register_nmi_handler() when in_nmi()

[Waiman Long <llong@redhat.com>]

On 11/29/24 11:57 AM, Thomas Gleixner wrote:
> On Thu, Nov 28 2024 at 20:55, Waiman Long wrote:
>> On 11/28/24 8:06 PM, Waiman Long wrote:
>>> On 11/28/24 4:28 AM, Peter Zijlstra wrote:
>>>> On Wed, Nov 27, 2024 at 06:34:55PM -0500, Waiman Long wrote:
>>>>> The __register_nmi_handler() function can be called in NMI context from
>>>>> nmi_shootdown_cpus() leading to a lockdep splat like the following.
>>>> This seems fundamentally insane. Why are we okay with this?
>>> According to the functional comment of nmi_shootdown_cpus(),
>>>
>>>   * nmi_shootdown_cpus() can only be invoked once. After the first
>>>   * invocation all other CPUs are stuck in crash_nmi_callback() and
>>>   * cannot respond to a second NMI.
>>>
>>> That is why it has to insert the crash_nmi_callback() call with
>>> register_nmi_handler() here in the NMI context. Changing this will
>>> require a fundamental redesign of the way this shutdown process need
>>> to be handled and I am not knowledgeable enough to do that. I will
>>> certainly appreciate idea to handle it in a more graceful way.
>> One idea that I current have is to add a emergency callback pointer to
>> the nmi_desc structure which, if set, has priority over the handlers in
>> the linked list and will be called first. In this way,
>> nmi_shootdown_cpus() can set the pointer to point to
>> crash_nmi_callback() without the need to take a lock and insert another
>> handler at the front of the list. Please let me know if this idea is
>> acceptable or not.
> That's way more sane.

Thanks for the feedback, I will work on a patch to do just that.

Cheers,
Longman