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

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Message-ID: <d135d6cc-5117-4b3a-9abe-2e5fd9e3e490@rivosinc.com>
Date: Tue, 24 Jun 2025 15:48:21 +0200
From: Clément Léger <cleger@...osinc.com>
To: Nam Cao <namcao@...utronix.de>
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,
 Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH] riscv: Enable interrupt during exception handling

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.

Hi Nam,

Yeah indeed, providing that all traps handlers really are reentrant, I
think it's fine.

Thanks,

Clément

> 
> Best regards,
> Nam