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

John Baldwin
2017-01-13T01:53:47Z

On Thursday, January 12, 2017 07:27:43 PM Luis Machado wrote:
> 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)?

Yes, that would work for me.

--
John Baldwin