Messages in this thread Patch in this message | | | Date | Sat, 7 Mar 2015 11:32:40 +0100 | From | Ingo Molnar <> | Subject | Re: [PATCH 1/1] x86/fpu: math_state_restore() should not blindly disable irqs |
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* Linus Torvalds <torvalds@linux-foundation.org> wrote:
> On Thu, Mar 5, 2015 at 11:58 PM, Ingo Molnar <mingo@kernel.org> 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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