Re: [PATCH] [PR gdb/23077] Fix error 'Couldn't get registers: Device busy' on FreeBSD

[Rajendra SY <rajendra dot sy at gmail dot com>]

Pedro,

I tested your patch it works on FreeBSD.
No error message after attach.


Thanks
Rajendra

On Fri, Apr 20, 2018 at 7:01 PM, Pedro Alves <palves@redhat.com> wrote:
> On 04/20/2018 10:46 AM, Rajendra SY wrote:
>> <patch attached>
>>
>> Problem:
>> GDB  "attach pid" command shows error message "Couldn't get registers:
>> Device busy" because of which below gdb tests fail.
>>
>> Cause:
>> On FreeBSD ptrace() syscall requires LWP id to fetch info from the
>> inferior process.
>> The current implementation is every command is executed completely and
>> then it wait's for asynchronous events.
>>
>> When attach command is executed it calls inf_ptrace_attach() function
>> which is currently doing just the ptrace(PT_ATTACH), it does not wait
>> for the process to stop to fetch LWP info.
>> The fbsd_wait() function is the one which fetches LWP info which is
>> being called after the command completes.
>>
>> Command execution code flow -
>> gdb_do_one_event() => .. => handle_file_event() => .. =>
>> execute_command() => check_frame_language_change()
>>
>> The async event handling code flow where LWP info gets updated -
>> gdb_one_event() => check_async_event_handlers() => ..
>> ->do_target_wait() => .. => fbsd_wait()
>>
>> The check, if the frame language changed via
>> check_frame_language_change() function is trying to read registers
>> using ptrace(pid) which fails because LWP id is not fetched.
>>
>> The purposed patch tries to avoid this situation by avoiding calling
>> check_frame_language_change() function just for the "attach" command.
>>
> Thanks for the patch, but we need to find a better fix.
>
> It seems to me that the problem is that we fail to mark
> the thread as executing between the initial attach, and the
> subsequent target_wait.  Can you give the attached patch a try?
> check_frame_language_change() then becomes a nop until
> the thread is marked not-executing again, after target_wait
> is called.
>
> We don't see the problem on Linux because there, the target_attach
> implementation waits for the thread to stop before returning.
> Still, that's supposedly hidden from the core, since the Linux
> target, like most targets, is a '!to_attach_no_wait' target.
>
> Pedro Alves