Messages in this thread | | | Subject | Re: [PATCH] arm64: use raw_smp_processor_id in stack backtrace dump | From | "Shi, Yang" <> | Date | Thu, 11 Feb 2016 09:36:52 -0800 |
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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 >
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