linux-kernel - Re: [PATCH] riscv: Enable interrupt during exception handling

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Message-ID: <20250624140815.GU1613200@noisy.programming.kicks-ass.net>
Date: Tue, 24 Jun 2025 16:08:15 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Clément Léger <cleger@...osinc.com>
Cc: Nam Cao <namcao@...utronix.de>, "Maciej W. Rozycki" <macro@...am.me.uk>,
	Palmer Dabbelt <palmer@...belt.com>,
	Paul Walmsley <paul.walmsley@...ive.com>, aou@...s.berkeley.edu,
	Alexandre Ghiti <alex@...ti.fr>, bigeasy@...utronix.de,
	clrkwllms@...nel.org, rostedt@...dmis.org,
	linux-rt-devel@...ts.linux.dev, linux-riscv@...ts.infradead.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] riscv: Enable interrupt during exception handling

On Tue, Jun 24, 2025 at 03:48:21PM +0200, Clément Léger wrote:
> 
> 
> On 24/06/2025 15:17, Nam Cao wrote:
> > On Tue, Jun 24, 2025 at 01:37:13PM +0200, Clément Léger wrote:
> >> On 24/06/2025 04:09, Maciej W. Rozycki wrote:
> >>> On Mon, 23 Jun 2025, Palmer Dabbelt wrote:
> >>>> I'm kind of split on a Fixes tag here.  One could argue it's a regression, as
> >>>> having interrupts disabled during exceptions is going to cause all sorts of
> >>>> performance issues for users.  Seems a bit risk to just backport, though...
> >>>>
> >>>> That said, if nobody noticed then it's probably a good sign nobody is really
> >>>> paying attention and we should just backport it before anyone notices...
> >>>
> >>>  Oh, someone did notice and it's not only performance, cf. 
> >>> <https://lore.kernel.org/r/alpine.DEB.2.21.2501070143250.18889@angie.orcam.me.uk/>.
> >>
> >> I also had a series which was doing so for misaligned accesses handling,
> >> but after discussion, it was not ok to do so.:
> >>
> >> https://lore.kernel.org/linux-riscv/20250422094419.GC14170@noisy.programming.kicks-ass.net/
> > 
> > If I understand that right, exceptions from kernel should be treated as
> > NMI, so that lockdep can tell us if exception handlers touch locks.
> > 
> > But (conditionally) enabling interrupts does not lose us that benefit. It
> > is still considered NMI by lockdep.
> > 
> > Unless I miss something, the patch is fine as is.

I'm confused, you're wanting to conditionally enable interrupts from a
kernel exception while its NMI like? *WHY* ?!