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Re: [PATCH 10/19] x86/dumpstack: add get_stack_info() interface

From Andy Lutomirski <luto@amacapital.net>
Newsgroups linux.kernel
Subject Re: [PATCH 10/19] x86/dumpstack: add get_stack_info() interface
Date 2016-07-23 01:30 +0200
Message-ID <rXS3g-3Rd-11@gated-at.bofh.it> (permalink)
References <rXtHz-4Eb-5@gated-at.bofh.it> <rXtHB-4Eb-45@gated-at.bofh.it>
Organization linux.* mail to news gateway

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On Thu, Jul 21, 2016 at 2:21 PM, Josh Poimboeuf <jpoimboe@redhat.com> wrote:
> valid_stack_ptr() is buggy: it assumes that all stacks are of size
> THREAD_SIZE, which is not true for exception stacks.  So the
> walk_stack() callbacks will need to know the location of the beginning
> of the stack as well as the end.
>
> Another issue is that in general the various features of a stack (type,
> size, next stack pointer, description string) are scattered around in
> various places throughout the stack dump code.

I finally figured out what visit_info is.  But would it make more
sense to track it in the unwind state so that the unwinder can
directly make sure it doesn't start looping?

And please remove test_and_set_bit() -- it's pointlessly slow.

> +static bool in_hardirq_stack(unsigned long *stack, struct stack_info *info,
> +                            unsigned long *visit_mask)
> +{
> +       unsigned long *begin = (unsigned long *)this_cpu_read(hardirq_stack);
> +       unsigned long *end   = begin + (THREAD_SIZE / sizeof(long));
> +
> +       if (stack < begin || stack >= end)
> +               return false;
> +
> +       if (visit_mask && test_and_set_bit(STACK_TYPE_IRQ, visit_mask))
> +               return false;
> +
> +       info->type      = STACK_TYPE_IRQ;
> +       info->begin     = begin;
> +       info->end       = end;
> +       info->next      = (unsigned long *)*begin;

This works, but it's a bit magic.  I don't suppose we could get rid of
this ->next thing entirely and teach show_stack_log_lvl(), etc. to
move from stack to stack by querying the stack type of whatever the
frame base address is if the frame base address ends up being out of
bounds for the current stack?  Or maybe the unwinder could even do
this by itself.

> +static bool in_exception_stack(unsigned long *s, struct stack_info *info,
> +                              unsigned long *visit_mask)
>  {
>         unsigned long stack = (unsigned long)s;
>         unsigned long begin, end;
> @@ -44,55 +63,62 @@ static unsigned long *in_exception_stack(unsigned long *s, char **name,
>                 if (stack < begin || stack >= end)
>                         continue;
>
> -               if (test_and_set_bit(k, visit_mask))
> +               if (visit_mask &&
> +                   test_and_set_bit(STACK_TYPE_EXCEPTION + k, visit_mask))
>                         return false;
>
> -               *name = exception_stack_names[k];
> -               return (unsigned long *)end;
> +               info->type      = STACK_TYPE_EXCEPTION + k;
> +               info->begin     = (unsigned long *)begin;
> +               info->end       = (unsigned long *)end;
> +               info->next      = (unsigned long *)info->end[-2];

This is so magical that I don't immediately see why it's correct.
Presumably it's because the thing two slots down from the top of the
stack is regs->sp?  If so, that needs a comment.

But again, couldn't we use the fact that we now know how to decode
pt_regs to avoid needing this?  I can imagine it being useful as a
fallback in the event that the unwinder fails, but this is just a
fallback.  Also, NMI is weird and I'm wondering whether this works at
all when trying to unwind from a looped NMI.

Fixing this up could be a followup after this series is in, I think --
you're preserving existing behavior AFAICS.  I just don't particularly
like the existing behavior.

FWIW, I think this code needs to be explicitly tested for the 32-bit
double fault case.  It's highly magical.

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[PATCH 00/19] x86/dumpstack: rewrite x86 stack dump code Josh Poimboeuf <jpoimboe@redhat.com> - 2016-07-21 23:30 +0200
  [PATCH 05/19] x86/dumpstack: fix function graph tracing stack dump reliability issues Josh Poimboeuf <jpoimboe@redhat.com> - 2016-07-21 23:30 +0200
  [PATCH 04/19] x86/dumpstack: make printk_stack_address() more generally useful Josh Poimboeuf <jpoimboe@redhat.com> - 2016-07-21 23:30 +0200
  [PATCH 18/19] x86/dumpstack: print stack identifier on its own line Josh Poimboeuf <jpoimboe@redhat.com> - 2016-07-21 23:30 +0200
  [PATCH 13/19] x86/stacktrace: convert save_stack_trace_*() to the new unwinder Josh Poimboeuf <jpoimboe@redhat.com> - 2016-07-21 23:30 +0200
  [PATCH 19/19] x86/dumpstack: print any pt_regs found on the stack Josh Poimboeuf <jpoimboe@redhat.com> - 2016-07-21 23:30 +0200
    Re: [PATCH 19/19] x86/dumpstack: print any pt_regs found on the stack Andy Lutomirski <luto@amacapital.net> - 2016-07-22 00:40 +0200
      Re: [PATCH 19/19] x86/dumpstack: print any pt_regs found on the stack Josh Poimboeuf <jpoimboe@redhat.com> - 2016-07-22 05:40 +0200
        Re: [PATCH 19/19] x86/dumpstack: print any pt_regs found on the stack Andy Lutomirski <luto@amacapital.net> - 2016-07-22 07:20 +0200
          Re: [PATCH 19/19] x86/dumpstack: print any pt_regs found on the stack Josh Poimboeuf <jpoimboe@redhat.com> - 2016-07-22 18:00 +0200
            Re: [PATCH 19/19] x86/dumpstack: print any pt_regs found on the stack Andy Lutomirski <luto@amacapital.net> - 2016-07-22 23:50 +0200
              Re: [PATCH 19/19] x86/dumpstack: print any pt_regs found on the stack Josh Poimboeuf <jpoimboe@redhat.com> - 2016-07-23 00:30 +0200
                Re: [PATCH 19/19] x86/dumpstack: print any pt_regs found on the stack Andy Lutomirski <luto@amacapital.net> - 2016-07-23 01:20 +0200
                Re: [PATCH 19/19] x86/dumpstack: print any pt_regs found on the stack Andy Lutomirski <luto@amacapital.net> - 2016-07-23 01:40 +0200
                Re: [PATCH 19/19] x86/dumpstack: print any pt_regs found on the stack Josh Poimboeuf <jpoimboe@redhat.com> - 2016-07-23 02:10 +0200
                Re: [PATCH 19/19] x86/dumpstack: print any pt_regs found on the stack Josh Poimboeuf <jpoimboe@redhat.com> - 2016-07-23 01:40 +0200
  [PATCH 08/19] x86/dumpstack: don't disable preemption in show_stack_log_lvl() and dump_trace() Josh Poimboeuf <jpoimboe@redhat.com> - 2016-07-21 23:30 +0200
  [PATCH 14/19] oprofile/x86: convert x86_backtrace() to the new unwinder Josh Poimboeuf <jpoimboe@redhat.com> - 2016-07-21 23:30 +0200
  [PATCH 06/19] x86/dumpstack: remove extra brackets around "EOE" Josh Poimboeuf <jpoimboe@redhat.com> - 2016-07-21 23:30 +0200
  [PATCH 10/19] x86/dumpstack: add get_stack_info() interface Josh Poimboeuf <jpoimboe@redhat.com> - 2016-07-21 23:30 +0200
    Re: [PATCH 10/19] x86/dumpstack: add get_stack_info() interface Andy Lutomirski <luto@amacapital.net> - 2016-07-23 01:30 +0200
      Re: [PATCH 10/19] x86/dumpstack: add get_stack_info() interface Andy Lutomirski <luto@amacapital.net> - 2016-07-23 02:00 +0200
        Re: [PATCH 10/19] x86/dumpstack: add get_stack_info() interface Josh Poimboeuf <jpoimboe@redhat.com> - 2016-07-23 15:10 +0200
      Re: [PATCH 10/19] x86/dumpstack: add get_stack_info() interface Josh Poimboeuf <jpoimboe@redhat.com> - 2016-07-23 02:00 +0200
        Re: [PATCH 10/19] x86/dumpstack: add get_stack_info() interface Andy Lutomirski <luto@amacapital.net> - 2016-07-23 02:20 +0200
          Re: [PATCH 10/19] x86/dumpstack: add get_stack_info() interface Josh Poimboeuf <jpoimboe@redhat.com> - 2016-07-23 16:10 +0200
  [PATCH 01/19] x86/dumpstack: remove show_trace() Josh Poimboeuf <jpoimboe@redhat.com> - 2016-07-21 23:30 +0200
    Re: [PATCH 01/19] x86/dumpstack: remove show_trace() Andy Lutomirski <luto@amacapital.net> - 2016-07-22 00:00 +0200
  [PATCH 12/19] perf/x86: convert perf_callchain_kernel() to the new unwinder Josh Poimboeuf <jpoimboe@redhat.com> - 2016-07-21 23:30 +0200
  [PATCH 02/19] x86/dumpstack: add get_stack_pointer() and get_frame_pointer() Josh Poimboeuf <jpoimboe@redhat.com> - 2016-07-21 23:30 +0200
    Re: [PATCH 02/19] x86/dumpstack: add get_stack_pointer() and get_frame_pointer() Andy Lutomirski <luto@amacapital.net> - 2016-07-22 00:00 +0200
  [PATCH 11/19] x86/dumptrace: add new unwind interface and implementations Josh Poimboeuf <jpoimboe@redhat.com> - 2016-07-21 23:30 +0200
  [PATCH 09/19] x86/dumpstack: simplify in_exception_stack() Josh Poimboeuf <jpoimboe@redhat.com> - 2016-07-21 23:30 +0200
    Re: [PATCH 09/19] x86/dumpstack: simplify in_exception_stack() Andy Lutomirski <luto@amacapital.net> - 2016-07-22 00:10 +0200
  [PATCH 07/19] x86/dumpstack: add IRQ_USABLE_STACK_SIZE define Josh Poimboeuf <jpoimboe@redhat.com> - 2016-07-21 23:30 +0200
    Re: [PATCH 07/19] x86/dumpstack: add IRQ_USABLE_STACK_SIZE define Andy Lutomirski <luto@amacapital.net> - 2016-07-22 00:10 +0200
      Re: [PATCH 07/19] x86/dumpstack: add IRQ_USABLE_STACK_SIZE define Josh Poimboeuf <jpoimboe@redhat.com> - 2016-07-22 03:50 +0200
        Re: [PATCH 07/19] x86/dumpstack: add IRQ_USABLE_STACK_SIZE define Ingo Molnar <mingo@kernel.org> - 2016-07-22 10:30 +0200
  Re: [PATCH 00/19] x86/dumpstack: rewrite x86 stack dump code Linus Torvalds <torvalds@linux-foundation.org> - 2016-07-23 02:30 +0200
    Re: [PATCH 00/19] x86/dumpstack: rewrite x86 stack dump code Andy Lutomirski <luto@amacapital.net> - 2016-07-23 02:40 +0200
      Re: [PATCH 00/19] x86/dumpstack: rewrite x86 stack dump code Josh Poimboeuf <jpoimboe@redhat.com> - 2016-07-23 07:40 +0200
        Re: [PATCH 00/19] x86/dumpstack: rewrite x86 stack dump code Linus Torvalds <torvalds@linux-foundation.org> - 2016-07-23 07:50 +0200
          Re: [PATCH 00/19] x86/dumpstack: rewrite x86 stack dump code Josh Poimboeuf <jpoimboe@redhat.com> - 2016-07-23 15:00 +0200

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