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

[Waiman Long <llong@redhat.com>]

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.

Cheers,
Longman