Re: [PATCH v4 01/39] task_work: Fix TWA_NMI_CURRENT error handling

[Peter Zijlstra]

On Wed, Jan 22, 2025 at 12:47:20PM -0800, Josh Poimboeuf wrote:
> On Wed, Jan 22, 2025 at 01:28:21PM +0100, Peter Zijlstra wrote:
> > On Tue, Jan 21, 2025 at 06:30:53PM -0800, Josh Poimboeuf wrote:
> > > It's possible for irq_work_queue() to fail if the work has already been
> > > claimed.  That can happen if a TWA_NMI_CURRENT task work is requested
> > > before a previous TWA_NMI_CURRENT IRQ work on the same CPU has gotten a
> > > chance to run.
> > 
> > I'm confused, if it fails then it's already pending, and we'll get the
> > notification already. You can still add the work.
> 
> Yeah, I suppose that makes sense.  If the pending irq_work is already
> going to set TIF_NOTIFY_RESUME anyway, there's no need to do that again.
> 
> > > The error has to be checked before the write to task->task_works.  Also
> > > the try_cmpxchg() loop isn't needed in NMI context.  The TWA_NMI_CURRENT
> > > case really is special, keep things simple by keeping its code all
> > > together in one place.
> > 
> > NMIs can nest,
> 
> Just for my understanding: for nested NMIs, the entry code basically
> queues up the next NMI, so the C handler (exc_nmi) can't nest.  Right?
> 
> > consider #DB (which is NMI like)
> 
> What exactly do you mean by "NMI like"?  Is it because a #DB might be
> basically running in NMI context, if the NMI hit a breakpoint?

No, #DB, #BP and such are considered NMI (and will have in_nmi() true)
because they can trigger anywhere, including sections where IRQs are
disabled.

> > doing task_work_add() and getting interrupted with NMI doing the same.
> 
> How exactly would that work?  At least with my patch the #DB wouldn't be
> able to use TWA_NMI_CURRENT unless in_nmi() were true

It is, see exc_debug_kernel() doing irqentry_nmi_enter().