Re: [PATCH 1/1] x86/fpu: math_state_restore() should not blindly disable irqs

[Ingo Molnar]

* Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:

> On Thu, Mar 5, 2015 at 11:58 PM, Ingo Molnar <mingo@xxxxxxxxxx> wrote:
> >
> > math_state_restore() was historically called with irqs disabled,
> > because that's how the hardware generates the trap, and also because
> > back in the days it was possible for it to be an asynchronous
> > interrupt and interrupt handlers run with irqs off.
> >
> > These days it's always an instruction trap, and furthermore it does
> > inevitably complex things such as memory allocation and signal
> > processing, which is not done with irqs disabled.
> >
> > So keep irqs enabled.
> 
> I agree with the "keep irqs enabled".
> 
> However, I do *not* agree with the actual patch, which doesn't do that at all.
> > @@ -844,8 +844,9 @@ void math_state_restore(void)
> >  {
> >         struct task_struct *tsk = current;
> >
> > +       local_irq_enable();
> > +
> 
> There's a big difference between "keep interrupts enabled" (ok) and
> "explicitly enable interrupts in random contexts" (*NOT* ok).

Agreed, so I thought that we already kind of did that:

   if (!tsk_used_math(tsk)) {
           local_irq_enable();

But yeah, my patch brought that to a whole new level by always doing 
it, without starting with adding a warning first.

> 
> So get rid of the "local_irq_enable()" entirely, and replace it with a
>
>    WARN_ON_ONCE(irqs_disabled());

Yeah, agreed absolutely - sorry about scaring (or annoying) you with a 
Signed-off-by patch, that was silly from me.

> and let's just fix the cases where this actually gets called with 
> interrupts off. [...]

Yes. I was a bit blinded by the 'easy to backport' aspect, so I 
concentrated on that, but it's more important to not break stuff.

> @@ -959,7 +949,7 @@ void __init trap_init(void)
>  	set_system_intr_gate(X86_TRAP_OF, &overflow);
>  	set_intr_gate(X86_TRAP_BR, bounds);
>  	set_intr_gate(X86_TRAP_UD, invalid_op);
> -	set_intr_gate(X86_TRAP_NM, device_not_available);
> +	set_trap_gate(X86_TRAP_NM, device_not_available);

So I wasn't this brave.

Historically modern x86 entry code ran with irqs off, because that's 
what the hardware gave us on most entry types. I'm not 100% sure we 
are ready to allow preemption of sensitive entry code on 
CONFIG_PREEMPT=y kernels. But we could try.

Thanks,

	Ingo
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