linux-kernel - Re: [PATCH] x86/fpu: Fix irq_fpu_usable() to return false during CPU onlining

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Message-ID: <20250524025559.GA68080@sol>
Date: Fri, 23 May 2025 19:55:59 -0700
From: Eric Biggers <ebiggers@...nel.org>
To: Thomas Gleixner <tglx@...utronix.de>
Cc: Ingo Molnar <mingo@...nel.org>, x86@...nel.org,
	linux-kernel@...r.kernel.org, linux-crypto@...r.kernel.org,
	linux-pm@...r.kernel.org, Borislav Petkov <bp@...en8.de>,
	Ayush Jain <Ayush.Jain3@....com>,
	Herbert Xu <herbert@...dor.apana.org.au>,
	Ard Biesheuvel <ardb@...nel.org>
Subject: Re: [PATCH] x86/fpu: Fix irq_fpu_usable() to return false during CPU
 onlining

On Wed, May 21, 2025 at 05:39:27PM +0200, Thomas Gleixner wrote:
> On Tue, May 20 2025 at 11:33, Ingo Molnar wrote:
> > * Eric Biggers <ebiggers@...nel.org> wrote:
> >
> >> Or we could use DEFINE_PER_CPU() = true in patch 1, then revert that 
> >> in patch 2 and replace it with the line in fpu__init_cpu().  But 
> >> again I think the split would be more likely to create problems than 
> >> solve them.
> >
> > Well, my request would be for the first patch to simply mimic current 
> > (and buggy) behavior as much as reasonably possible (obviously the 
> > effects of BSS zeroing shouldn't be mimiced 100%) - and the second 
> > patch to fix the initialization-ordering bug.
> 
> So the first patch is then incomprehensible buggy and needs a trivial
> one-liner to fix up, right?
> 
> TBH, that's just bonkers. Eric's patch is trivial enough as is and easy
> to review. Artifical patch splitting with buggy intermediate state makes
> only sense, when the overall changes are massive and hard to
> review. That's absolutely not the case here.
> 
> Thanks,

That sounds reasonable to me.  Anyway, any interest in applying one of the
versions to the x86 tree?  Maybe either this original one, or v2 which has the
extra WARN_ON_FPU() checks that Ingo requested.

- Eric