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

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Message-ID: <20250624131730.XqPd1HUR@linutronix.de>
Date: Tue, 24 Jun 2025 15:17:30 +0200
From: Nam Cao <namcao@...utronix.de>
To: Clément Léger <cleger@...osinc.com>
Cc: "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 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.

Best regards,
Nam