linux-kernel - Re: [PATCH 3/3] x86/fpu: Don't support kernel-mode FPU when irqs

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Message-ID: <CAMj1kXGe0hMD-71KYN_htJztAL+P8vFNf+9+W_aVDkHx3nCEWA@mail.gmail.com>
Date: Mon, 19 May 2025 11:49:28 +0200
From: Ard Biesheuvel <ardb@...nel.org>
To: Ingo Molnar <mingo@...nel.org>
Cc: Eric Biggers <ebiggers@...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>, Thomas Gleixner <tglx@...utronix.de>, Ayush Jain <Ayush.Jain3@....com>, 
	Herbert Xu <herbert@...dor.apana.org.au>
Subject: Re: [PATCH 3/3] x86/fpu: Don't support kernel-mode FPU when irqs_disabled()

On Mon, 19 May 2025 at 10:06, Ingo Molnar <mingo@...nel.org> wrote:
>
>
> * Eric Biggers <ebiggers@...nel.org> wrote:
>
> > > # echo PANIC > /sys/kernel/debug/provoke-crash/DIRECT
> > >
> > > Another case that likely executes with IRQs disabled (but I haven't
> > > double checked) is reset_system(), which may return with an error, or
> > > reboot/poweroff the machine and never return.
> >
> > That makes sense to me.  preempt_disable() and preempt_enable() are already
> > allowed when IRQs are disabled, and I'm not sure why local_bh_disable() and
> > local_bh_enable() are different.
>
> Because local_bh_enable() may run softirq handlers immediately if
> there's pending softirqs, which shouldn't be done in hardirq context.
>

Sure, but why is that mandatory?

preempt_disable() has preempt_enable() and preempt_enable_no_resched()
counterparts. Could we have a local_bh_enable_no_xxx() version that
re-enables async softirq processing on the current CPU but does not
kick off a synchronous processing run?