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

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Message-ID: <335675da8a85e26874c5847760455a1c01097390.camel@redhat.com>
Date: Thu, 24 Oct 2024 15:12:11 -0400
From: Lyude Paul <lyude@...hat.com>
To: Peter Zijlstra <peterz@...radead.org>, Thomas Gleixner
 <tglx@...utronix.de>
Cc: Boqun Feng <boqun.feng@...il.com>, Dirk Behme <dirk.behme@...il.com>, 
 rust-for-linux@...r.kernel.org, Danilo Krummrich <dakr@...hat.com>, 
 airlied@...hat.com, Ingo Molnar <mingo@...hat.com>, will@...nel.org, Waiman
 Long <longman@...hat.com>, linux-kernel@...r.kernel.org, Miguel Ojeda
 <ojeda@...nel.org>, Alex Gaynor <alex.gaynor@...il.com>,
 wedsonaf@...il.com,  Gary Guo <gary@...yguo.net>, Björn
 Roy Baron <bjorn3_gh@...tonmail.com>,  Benno Lossin
 <benno.lossin@...ton.me>, Andreas Hindborg <a.hindborg@...sung.com>,
 aliceryhl@...gle.com,  Trevor Gross <tmgross@...ch.edu>
Subject: Re: [POC 1/6] irq & spin_lock: Add counted interrupt
 disabling/enabling

On Thu, 2024-10-24 at 12:05 +0200, Peter Zijlstra wrote:
> 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.

Completely agree. Plus, more often then not even in a substantially
complicated piece of code that's dealing with the interrupt state, it's not
common to have that many nest levels because critical sections like that
should be small and self-contained anyhow.

> 
> > 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*.
> 

-- 
Cheers,
 Lyude Paul (she/her)
 Software Engineer at Red Hat

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