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

[Ingo Molnar <mingo@kernel.org>]

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* 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