Re: Thread Specific Architectures And Python Unwinder API

[Andrew Burgess <aburgess@redhat.com>]

Luis Machado <luis.machado@arm.com> writes:

> On 10/12/23 14:02, Andrew Burgess wrote:
>> Luis Machado <luis.machado@arm.com> writes:
>> 
>>> On 10/11/23 11:06, Luis Machado wrote:
>>>> Hi Andrew,
>>>>
>>>> On 10/11/23 09:47, Andrew Burgess wrote:
>>>>>
>>>>> Hi Luis,
>>>>>
>>>>> While working on something else I was looking at the Python Unwinder API
>>>>> code, and I suspect that the thread-specific architectures support might
>>>>> (currently) break the Python Unwinder support.
>>>>>
>>>>> If it is, then I think the fix is pretty simple, but before I posted it,
>>>>> I wondered if you could confirm that things are indeed, currently
>>>>> broken.
>>>>>
>>>>> Attached at the end of this email is a Python unwinder.  You'll need to
>>>>> supply your own test program that makes use of sve/sme, and thus uses
>>>>> thread-specific architectures.
>>>>>
>>>>> What you'll need to do is:
>>>>>
>>>>>   $ gdb -q test_file_that_uses_sve_sme
>>>>>   Reading symbols from .... etc ...
>>>>>   (gdb) source ./unwinder.py
>>>>>   (gdb) break function_where_a_thread_specific_arch_will_be_in_use
>>>>>   Breakpoint 1 at ... etc ...
>>>>>   (gdb) run
>>>>>   Starting program: ... etc ...
>>>>>
>>>>> Now at this point, when you stop, you should see at least one instance
>>>>> of the banner:
>>>>>   
>>>>>   ***********************************
>>>>>   * Have executed the test unwinder *
>>>>>   ***********************************
>>>>>
>>>>> being printed, probably more.  As you step though the function you
>>>>> should see more instances of the banner being printed.
>>>>>
>>>>> To reveal the bug then it is important that when GDB stops in
>>>>> function_where_a_thread_specific_arch_will_be_in_use, the per-thread
>>>>> gdbarch that it creates _must_ be different from the inferior wide,
>>>>> top-level gdbarch.
>>>>>
>>>>> If you don't see the banner then my suspicion is correct, and the Python
>>>>> Unwinder API was broken when the thread-specific architecture support
>>>>> was added.
>>>
>>> I've confirmed I don't see the banner. So this seems to be broken.
>>>
>>> Is the fix to set the python unwinder hook elsewhere? Or maybe save that
>>> information when we're trying to pick a different gdbarch?
>> 
>> I'll post my patch for this later today.  But I'm going to propose
>> changing the 'architecture_changed' observable into a 'new_architecture'
>> observable (which is called in a different place).  As such, every new
>> architecture will have the Python unwinder registered correctly.
>> 
>> I just need to update the commit message with some details of this issue
>> as justification for the change, and I'll post it -- it's a pretty small
>> change.  I'll CC you on the new thread.
>
> Great. Thanks for the heads-up. Let me know if you need some more testing.

If you're keen, the series is now posted here:

  https://inbox.sourceware.org/gdb-patches/f5efeaed8c7150294024f5d2f6d98b748fd6bf56.1697120493.git.aburgess@redhat.com/T/#u

If you apply this, and then repeat the previous experiment (loading the
dummy Python unwinder), you should now see the banner being printed.

Thanks,
Andrew