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

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?

> 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 due to NMI hitting
a breakpoint.  In which case a nested NMI wouldn't actually nest, it
would get "queued" by the entry code.

But yeah, I do see how the reverse can be true: somebody sets a
breakpoint in task_work, right where it's fiddling with the list head.
NMI calls task_work_add(TWA_NMI_CURRENT), triggering the #DB, which also
calls task_work_add().

-- 
Josh