Re: [PATCH] PR threads/20743: Don't attempt to suspend or resume exited threads.

[Luis Machado <lgustavo@codesourcery.com>]

On 01/12/2017 01:16 PM, John Baldwin wrote:
> On Thursday, January 12, 2017 10:29:00 AM Luis Machado wrote:
>> On 12/28/2016 11:37 AM, John Baldwin wrote:
>>> On Wednesday, December 28, 2016 09:07:07 AM Vasil Dimov wrote:
>>>> On Tue, Dec 27, 2016 at 13:03:27 -0800, John Baldwin wrote:
>>>> [...]
>>>>> I have tried changing fbsd_wait() to return a TARGET_WAITKIND_SPURIOUS
>>>>> instead of explicitly continuing the process, but that doesn't help, and it
>>>>> means that the ptid being returned is still T1 in that case.
>>>>>
>>>>> I'm not sure if I should explicitly be calling delete_exited_threads() in
>>>>> fbsd_resume() before calling iterate_threads()?  Alternatively, fbsd_resume()
>>>>> could use ALL_NONEXITED_THREADS() instead of iterate_threads() (it isn't
>>>>> clear to me which of these is preferred since both are in use).
>>>>>
>>>>> I added the assertion for my own sanity.  I suspect gdb should never try to
>>>>> invoke target_resume() with a ptid of an exited thread, but if for some
>>>>> reason it did the effect on FreeBSD would be a hang since we would suspend
>>>>> all the other threads and when the process was continued via PT_CONTINUE it
>>>>> would have nothing to do and would never return from wait().  I'd rather have
>>>>> gdb fail an assertion in that case rather than hang.
>>>> [...]
>>>>
>>>> Hi,
>>>>
>>>> I am not sure if this is related, but since I get a hang I would rather
>>>> mention it: with the John's patch (including the assert) gdb does not
>>>> emit the "ptrace: No such process" error, but when I attempt to quit,
>>>> it hangs:
>>>
>>> No, this is a separate bug in the kernel whereby a process doesn't
>>> treat PT_KILL as a detach-like event but incorrectly expects to keep
>>> getting PT_CONTINUE events for a while until it finally exits.  I'm
>>> working on writing up regression/unit tests for PT_KILL and then
>>> fixing the bug.
>>>
>>
>> I think the patch is mainly papering over a bigger problem. My guess is
>> that the native fbsd backend is not doing something it should.
>>
>> I'd check how linux-nat.c is doing things and then try to confirm the
>> fbsd behavior is sane.
>>
>> For example, i noticed linux-nat.c has exit_lwp (...) that handles
>> deletion of both thread information and the thread itself (lwp). Even if
>> it is the currently-selected thread, we *will* get the lwp removed from
>> the list of existing lwp's.
>
> FreeBSD's backend doesn't maintain a separate lwp list, it just uses
> the existing GDB thread list.  For FreeBSD's backend the two lists would
> simply mirror each other so it seems a bit of a waste to maintain a
> duplicate list.  exit_lwp() calls delete_thread() which is the same thing
> the FreeBSD backend is doing, so if that is the current thread in
> inferior_ptid, the Linux backend will also being leaving the exited
> thread around in GDB's list until some future call to delete_exited_threads().
>
> I think the thing that makes Linux work is that it doesn't use GDB's
> thread list.  Meaning, it doesn't walk over GDB's thread list, but instead
> iterates over its private LWP list via iterate_over_lwps().  It would seem
> that GDB's thread list is designed so that backends shouldn't need their
> own thread list (you can add target-specific data with a custom destructor
> that gets invoked when freeing a thread for example), but the Linux backend
> doesn't choose to use it that way?
>
> Looking at some other threaded backends:
>
> - aix-thread.c relies on custom ptrace ops that resume a single thread
> - darwin-nat.c uses its own thread list (stored in the inferior's
>   private data) instead of GDB's thread list.
> - gnu-nat.c uses its own thread list instead of GDB's thread list.
> - obsd-nat.c uses GDB's thread list but doesn't seem to support resuming
>   individual threads (only entire processes).
> - procfs.c maintains its own thread list, but it doesn't seem to use it
>   for resume but relies on the associated kernel resuming either an
>   entire process or a single thread in a process via different ioctls.
> - remote.c:remote_resume() uses ALL_NON_EXITED_THREADS
> - windows_nat.cwindows_resume() calls windows_continue() which uses a
>   target-internal thread list rather than GDB's thread list.
>
>> It doesn't make sense to keep a thread that has already exitted in the
>> list of threads we are manipulating.
>
> FreeBSD's backend isn't making that choice.  delete_thread() in threads.c
> is the one making that choice.  If FreeBSD's backend were to define its
> own thread list, the contents would be identical except it would not
> include any exited threads, so skipping exited threads gives the same
> result as walking a hypothetical private list.
>

So i take it using ALL_NON_EXITED_THREADS is something that would seem 
reasonable to use in this case and not iterate through all threads (even 
ones marked exitting)?