linux-kernel - Re: [PATCH] x86/nmi: Use trylock in __register_nmi_handler() when in

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Message-ID: <89cdf387-f75f-472f-9f4b-e3582d1d2c93@redhat.com>
Date: Thu, 28 Nov 2024 20:55:50 -0500
From: Waiman Long <llong@...hat.com>
To: Peter Zijlstra <peterz@...radead.org>
Cc: Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>,
 Borislav Petkov <bp@...en8.de>, Dave Hansen <dave.hansen@...ux.intel.com>,
 x86@...nel.org, linux-kernel@...r.kernel.org, "H. Peter Anvin"
 <hpa@...or.com>
Subject: Re: [PATCH] x86/nmi: Use trylock in __register_nmi_handler() when
 in_nmi()

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