Re: [PATCH 3/3] x86/fpu: Don't support kernel-mode FPU when irqs_disabled()

Ingo Molnar
2025-05-19T08:06:03Z

* Eric Biggers <ebiggers@kernel.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.

This is a key optimization of the Linux networking code, which uses
BH-off/BH-on sections instead of IRQS-off/IRQS-on critical sections,
for performance reasons.

Thanks,

Ingo