Re: [POC 1/6] irq & spin_lock: Add counted interrupt disabling/enabling

[Peter Zijlstra <peterz@infradead.org>]

On Wed, Oct 23, 2024 at 10:38:38PM +0200, Thomas Gleixner wrote:
> On Wed, Oct 23 2024 at 21:51, Peter Zijlstra wrote:
> > On Wed, Oct 23, 2024 at 09:34:27PM +0200, Thomas Gleixner wrote:
> >> On Thu, Oct 17 2024 at 22:51, Boqun Feng wrote:
> >> Ideally you make that part of the preemption count. Bit 24-30 are free
> >> (or we can move them around as needed). That's deep enough and you get
> >> the debug sanity checking of the preemption counter for free (might need
> >> some extra debug for this...)
> >
> > Urgh, so we've already had trouble that nested spinlocks bust through
> > the 0xff preempt mask (because lunacy).
> 
> Seriously? Such overflow should just panic the kernel. That's broken by
> definition.

It will not panic, it will mostly work and randomly do weird things.
Only once you build with DEBUG_PREEMPT=y will you notice.

> > You sure you want to be this stingy with bits?
> 
> Anything above 64 nest levels is beyond insane.

Agreed.

> But if we want to support insanity then we make preempt count 64 bit and
> be done with it. But no, I don't think that encouraging insanity is a
> good thing.

The problem is that in most release builds the overflow will be silent
and cause spurious weirdness that is a pain in the arse to debug :/

That is my only concern -- making insane code crash hard is good, making
it silently mostly work but cause random weirdness is not.

> It actually makes a lot of sense even for the non rust case to avoid
> local_irq_save/restore. We discussed that for years and I surely have
> some half finished patch set to implement it somewhere in the poison
> cabinet.

Heh, yeah, me too. I even have patches using CR8 *somewhere*.