Re: [lkp] [x86/fpu] 58122bf1d8: WARNING: CPU: 0 PID: 1 at arch/x86/include/asm/fpu/internal.h:529 fpu__restore+0x28f/0x9ab()

Ingo Molnar
2016-02-27T12:02:18Z

* Borislav Petkov <bp@alien8.de> wrote:

> From: Borislav Petkov <bp@suse.de>
> Date: Mon, 15 Feb 2016 19:50:33 +0100
> Subject: [PATCH] x86/FPU: Fix double FPU regs activation
>
> sys_sigreturn() calls fpu__restore_sig() with interrupts enabled. When
> restoring a 32-bit signal frame. And it can happen that we get preempted
> right after setting ->fpstate_active in a task's FPU.
>
> After we get preempted, we switch between tasks merrily and eventually
> are about to switch to that task above whose ->fpstate_active we
> set. We enter __switch_to() and do switch_fpu_prepare(). Our task gets
> ->fpregs_active set, we find ourselves back on the call stack below and
> especially in __fpu__restore_sig() which sets ->fpregs_active again.
>
> Leading to that whoops below.
>
> So let's enlarge the preemption-off region so that we set
> ->fpstate_active with preemption disabled and thus not trigger
> fpu.preload:
>
> switch_fpu_prepare
>
> ...
>
> fpu.preload = static_cpu_has(X86_FEATURE_FPU) &&
> new_fpu->fpstate_active &&
> ^^^^^^^^^^^^^^^^^^^^^^
>
> prematurely.

So I'm wondering, why did this commit:

58122bf1d856 x86/fpu: Default eagerfpu=on on all CPUs

trigger the warning, while it never triggered on CPUs that were already
eagerfpu=on for years?

There must be something we are still missing I think.

Thanks,

Ingo