Thread Specific Architectures And Python Unwinder API

Luis Machado
Thu Oct 12 13:11:51 GMT 2023

On 10/12/23 14:02, Andrew Burgess wrote:
> Luis Machado <> 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 ./
>>>>   (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.

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