Date:	Tue, 2 Sep 2014 00:04:18 -0700
From:	Suresh Siddha <sbsiddha@...il.com>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Al Viro <viro@...iv.linux.org.uk>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Fenghua Yu <fenghua.yu@...el.com>,
	Bean Anderson <bean@...lsystems.com>,
	"H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	"the arch/x86 maintainers" <x86@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 4/4] x86, fpu: irq_fpu_usable: kill all checks except !in_kernel_fpu

On Fri, Aug 29, 2014 at 11:17 AM, Oleg Nesterov <oleg@...hat.com> wrote:
> ONCE AGAIN, THIS IS MORE THE QUESTION THAN THE PATCH.

this patch I think needs more thought for sure. please see below.

>
> interrupted_kernel_fpu_idle() does:
>
>         if (use_eager_fpu())
>                 return true;
>
>         return !__thread_has_fpu(current) &&
>                 (read_cr0() & X86_CR0_TS);
>
> and it is absolutely not clear why these 2 cases differ so much.
>
> To remind, the use_eager_fpu() case is buggy; __save_init_fpu() in
> __kernel_fpu_begin() can race with math_state_restore() which does
> __thread_fpu_begin() + restore_fpu_checking(). So we should fix this
> race anyway and we can't require __thread_has_fpu() == F likes the
> !use_eager_fpu() case does, in this case kernel_fpu_begin() will not
> work if it interrupts the idle thread (this will reintroduce the
> performance regression fixed by 5187b28f).
>
> Probably math_state_restore() can use kernel_fpu_disable/end() which
> sets/clears in_kernel_fpu, or it can disable irqs. Doesn't matter, we
> should fix this bug anyway.
>
> And if we fix this bug, why else !use_eager_fpu() case needs the much
> more strict check? Why we can't handle the __thread_has_fpu(current)
> case the same way?
>
> The comment deleted by this change says:
>
>         and TS must be set so that the clts/stts pair does nothing
>
> and can explain the difference, but I can not understand this (again,
> assuming that we fix the race(s) mentoined above).
>
> Say, user_fpu_begin(). Yes, kernel_fpu_begin/end() can restore X86_CR0_TS.
> But this should be fine?

No. The reason is that has_fpu state and cr0.TS can get out of sync.

Let's say you get an interrupt after clts() in __thread_fpu_begin()
called as part of user_fpu_begin().

And because of this proposed change, irq_fpu_usable() returns true and
an interrupt can end-up using fpu and after the return from interrupt
we can have a state where cr0.TS is set but we end up resuming the
execution from __thread_set_has_fpu(). Now after this point has_fpu is
set but cr0.TS is set. And now any schedule() with this state (let's
say immd after preemption_enable() at the end of user_fpu_begin()) is
dangerous. We can get a dna fault in the middle of __switch_to() which
can lead to subtle bugs.


> A context switch before restore_user_xstate()
> can equally set it back?
> And device_not_available() should be fine even
> in kernel context?

not in some critical places like switch_to().

other than this patch, rest of the changes look ok to me. Can you
please resend this patchset with the math_state_restore() race
addressed aswell?

thanks,
suresh

>
> I'll appreciate any comment.
> ---
>  arch/x86/kernel/i387.c |   44 +-------------------------------------------
>  1 files changed, 1 insertions(+), 43 deletions(-)
>
> diff --git a/arch/x86/kernel/i387.c b/arch/x86/kernel/i387.c
> index 9fb2899..ef60f33 100644
> --- a/arch/x86/kernel/i387.c
> +++ b/arch/x86/kernel/i387.c
> @@ -22,54 +22,12 @@
>  static DEFINE_PER_CPU(bool, in_kernel_fpu);
>
>  /*
> - * Were we in an interrupt that interrupted kernel mode?
> - *
> - * On others, we can do a kernel_fpu_begin/end() pair *ONLY* if that
> - * pair does nothing at all: the thread must not have fpu (so
> - * that we don't try to save the FPU state), and TS must
> - * be set (so that the clts/stts pair does nothing that is
> - * visible in the interrupted kernel thread).
> - *
> - * Except for the eagerfpu case when we return 1.
> - */
> -static inline bool interrupted_kernel_fpu_idle(void)
> -{
> -       if (this_cpu_read(in_kernel_fpu))
> -               return false;
> -
> -       if (use_eager_fpu())
> -               return true;
> -
> -       return !__thread_has_fpu(current) &&
> -               (read_cr0() & X86_CR0_TS);
> -}
> -
> -/*
> - * Were we in user mode (or vm86 mode) when we were
> - * interrupted?
> - *
> - * Doing kernel_fpu_begin/end() is ok if we are running
> - * in an interrupt context from user mode - we'll just
> - * save the FPU state as required.
> - */
> -static inline bool interrupted_user_mode(void)
> -{
> -       struct pt_regs *regs = get_irq_regs();
> -       return regs && user_mode_vm(regs);
> -}
> -
> -/*
>   * Can we use the FPU in kernel mode with the
>   * whole "kernel_fpu_begin/end()" sequence?
> - *
> - * It's always ok in process context (ie "not interrupt")
> - * but it is sometimes ok even from an irq.
>   */
>  bool irq_fpu_usable(void)
>  {
> -       return !in_interrupt() ||
> -               interrupted_user_mode() ||
> -               interrupted_kernel_fpu_idle();
> +       return !this_cpu_read(in_kernel_fpu);
>  }
>  EXPORT_SYMBOL(irq_fpu_usable);
>
> --
> 1.5.5.1
>
>
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