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

[Waiman Long]

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