Re: Thread Specific Architectures And Python Unwinder API

[Andrew Burgess <aburgess@redhat.com>]

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.

Thanks,
Andrew