Re: [PATCH] arm64: use raw_smp_processor_id in stack backtrace dump

[Shi, Yang]

On 2/11/2016 2:41 AM, James Morse wrote:
> Hi!
>
> On 10/02/16 18:12, Shi, Yang wrote:
> > On 2/10/2016 4:10 AM, Will Deacon wrote:
> > > On Wed, Feb 10, 2016 at 11:52:31AM +0000, James Morse wrote:
> > > > On 10/02/16 10:29, Will Deacon wrote:
> > > > > On Tue, Feb 09, 2016 at 01:26:22PM -0800, Yang Shi wrote:
> > > > > > dump_backtrace may be called in kthread context, which is not bound to a
> > > > > > single
> > > > > > cpu, i.e. khungtaskd, then calling smp_processor_id may trigger the below bug
> > > > > > report:
> > > > >
> > > > > If we're preemptible here, it means that our irq_stack_ptr is potentially
> > > > > bogus. Whilst this isn't an issue for kthreads, it does feel like we
> > > > > could make this slightly more robust in the face of potential frame
> > > > > corruption. Maybe just zero the IRQ stack pointer if we're in preemptible
> > > > > context?
> > > >
> > > > Switching between stacks is only valid if we are tracing ourselves while on the
> > > > irq_stack, we should probably prevent it for other tasks too.
> > > >
> > > > Something like (untested):
> > > > ---------------------
> > > > if (tsk == current && in_atomic())
> > > >      irq_stack_ptr = IRQ_STACK_PTR(smp_processor_id());
> >
> > One follow up question, is it possible to have both tsk != current and
> > on_irq_stack is true at the same time?
>
> No. If you are tracing an irq stack, it must be your own stack.
>
> If this weren't the case, it would be the stack of a running task on a remote
> CPU, and you would be racing with the remote CPU changing the values you are
> reading. Fortunately nothing tries to do this.
>
> (The third case would be tracing a sleeping irq stack - this doesn't happen
>   either, as we switch back to the original stack before calling schedule()).
>
>
> > If it is possible, this may be a problem
> > in unwind_frame called by profile_pc which has tsk being NULL.
>
> Ah, well spotted. I guess there should also be a != NULL comparison thrown into
> the mix. I don't think it will be a problem for profile_pc() as it should always
> find a !in_lock_functions() frame before it needs to switch stack, (which we are
> preventing it from doing). If this ever did happen, it will return 0.

Thanks for the elaboration. I changed the logic a little bit to:

if (tsk == current && !preemptible())
                irq_stack_ptr = IRQ_STACK_PTR(smp_processor_id());
else
                irq_stack_ptr = 0;

In this way, the NULL pointer will be covered by "else" too.

v2 patch will be sent out soon once I'm done some smoke testing.

Yang

> Thanks,
>
> James