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Re: WARNING: CPU: 0 PID: 3031 at ./arch/x86/include/asm/fpu/internal.h:530 fpu__restore+0x90/0x130()

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* Andy Lutomirski <luto@amacapital.net> wrote:

> On Feb 15, 2016 12:14 PM, "Borislav Petkov" <bp@alien8.de> wrote:
> >
> > ---
> > From: Borislav Petkov <bp@suse.de>
> > Date: Mon, 15 Feb 2016 19:50:33 +0100
> > Subject: [RFC PATCH] x86/FPU: Fix double FPU regs activation
> >
> > On the entry_INT80_32->do_syscall_32_irqs_on path on 32-bit we run with
> > interrupts enabled.
> 
> I would change this a little bit.
> 
> sys_sigreturn calls fpu__restore_sig with interrupts enabled.  When
> restoring a 32-bit signal frame, it can happen that...
> 
> > 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 I'm wondering why this started triggering only now. Is this a pre-existing bug 
that somehow got triggered via:

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

? If yes then we need a plausible theory of how that never triggered on modern 
Intel CPUs that had eagerfpu enabled for years.

Or perhaps was it caused by one of the other changes in tip:x86/fpu:

  c6ab109f7e0e x86/fpu: Speed up lazy FPU restores slightly
  a20d7297045f x86/fpu: Fold fpu_copy() into fpu__copy()
  5ed73f40735c x86/fpu: Fix FNSAVE usage in eagerfpu mode
  4ecd16ec7059 x86/fpu: Fix math emulation in eager fpu mode

?

Which would make this a recently introduced regression.

Thanks,

	Ingo

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WARNING: CPU: 0 PID: 3031 at  ./arch/x86/include/asm/fpu/internal.h:530 fpu__restore+0x90/0x130() Borislav Petkov <bp@alien8.de> - 2016-02-11 20:30 +0100
  Re: WARNING: CPU: 0 PID: 3031 at ./arch/x86/include/asm/fpu/internal.h:530  fpu__restore+0x90/0x130() Andy Lutomirski <luto@amacapital.net> - 2016-02-12 00:50 +0100
    Re: WARNING: CPU: 0 PID: 3031 at ./arch/x86/include/asm/fpu/internal.h:530  fpu__restore+0x90/0x130() Andy Lutomirski <luto@amacapital.net> - 2016-02-12 02:20 +0100
      Re: WARNING: CPU: 0 PID: 3031 at  ./arch/x86/include/asm/fpu/internal.h:530 fpu__restore+0x90/0x130() Borislav Petkov <bp@alien8.de> - 2016-02-12 18:10 +0100
        Re: WARNING: CPU: 0 PID: 3031 at  ./arch/x86/include/asm/fpu/internal.h:530 fpu__restore+0x90/0x130() Borislav Petkov <bp@alien8.de> - 2016-02-15 20:20 +0100
          Re: WARNING: CPU: 0 PID: 3031 at ./arch/x86/include/asm/fpu/internal.h:530  fpu__restore+0x90/0x130() Andy Lutomirski <luto@amacapital.net> - 2016-02-16 03:30 +0100
            Re: WARNING: CPU: 0 PID: 3031 at  ./arch/x86/include/asm/fpu/internal.h:530 fpu__restore+0x90/0x130() Ingo Molnar <mingo@kernel.org> - 2016-02-17 09:20 +0100
              Re: WARNING: CPU: 0 PID: 3031 at  ./arch/x86/include/asm/fpu/internal.h:530 fpu__restore+0x90/0x130() Borislav Petkov <bp@alien8.de> - 2016-02-17 10:30 +0100
                Re: WARNING: CPU: 0 PID: 3031 at  ./arch/x86/include/asm/fpu/internal.h:530 fpu__restore+0x90/0x130() Ingo Molnar <mingo@kernel.org> - 2016-02-17 10:40 +0100
                Re: WARNING: CPU: 0 PID: 3031 at  ./arch/x86/include/asm/fpu/internal.h:530 fpu__restore+0x90/0x130() Borislav Petkov <bp@alien8.de> - 2016-02-17 11:40 +0100
                Re: WARNING: CPU: 0 PID: 3031 at  ./arch/x86/include/asm/fpu/internal.h:530 fpu__restore+0x90/0x130() Ingo Molnar <mingo@kernel.org> - 2016-02-17 12:10 +0100
                Re: WARNING: CPU: 0 PID: 3031 at  ./arch/x86/include/asm/fpu/internal.h:530 fpu__restore+0x90/0x130() Borislav Petkov <bp@alien8.de> - 2016-02-17 12:50 +0100
                Re: WARNING: CPU: 0 PID: 3031 at ./arch/x86/include/asm/fpu/internal.h:530  fpu__restore+0x90/0x130() Andy Lutomirski <luto@amacapital.net> - 2016-02-17 19:00 +0100
      Re: WARNING: CPU: 0 PID: 3031 at  ./arch/x86/include/asm/fpu/internal.h:530 fpu__restore+0x90/0x130() Borislav Petkov <bp@alien8.de> - 2016-02-15 20:10 +0100
    Re: WARNING: CPU: 0 PID: 3031 at  ./arch/x86/include/asm/fpu/internal.h:530 fpu__restore+0x90/0x130() Borislav Petkov <bp@alien8.de> - 2016-02-12 12:20 +0100

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