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

[John Baldwin <jhb at freebsd dot org>]

On Tuesday, December 27, 2016 01:03:27 PM John Baldwin wrote:
> On Tuesday, December 27, 2016 05:43:29 PM Vasil Dimov wrote:
> > On Fri, Dec 23, 2016 at 15:43:19 -0600, Luis Machado wrote:
> > > On 12/23/2016 03:28 PM, John Baldwin wrote:
> > > > When resuming a native FreeBSD process, ignore exited threads when
> > > > suspending/resuming individual threads prior to continuing the process.
> > > >
> > > > gdb/ChangeLog:
> > > >
> > > > 	PR threads/20743
> > > > 	* fbsd-nat.c (resume_one_thread_cb): Ignore exited threads.
> > > > 	(resume_all_threads_cb): Likewise.
> > > > 	(fbsd_resume): Assert resuming thread has not exited.
> > > > ---
> > > >  gdb/ChangeLog  | 7 +++++++
> > > >  gdb/fbsd-nat.c | 7 +++++++
> > > >  2 files changed, 14 insertions(+)
> > > >
> > > > diff --git a/gdb/ChangeLog b/gdb/ChangeLog
> > > > index db6e913..4fb3732 100644
> > > > --- a/gdb/ChangeLog
> > > > +++ b/gdb/ChangeLog
> > > > @@ -1,3 +1,10 @@
> > > > +2016-12-23  John Baldwin  <jhb@FreeBSD.org>
> > > > +
> > > > +	PR threads/20743
> > > > +	* fbsd-nat.c (resume_one_thread_cb): Ignore exited threads.
> > > > +	(resume_all_threads_cb): Likewise.
> > > > +	(fbsd_resume): Assert resuming thread has not exited.
> > > > +
> > > >  2016-12-22  Doug Evans  <xdje42@gmail.com>
> > > >
> > > >  	* infrun.c (set_step_over_info): Add comment.
> > > > diff --git a/gdb/fbsd-nat.c b/gdb/fbsd-nat.c
> > > > index ade62f1..7cd08c6 100644
> > > > --- a/gdb/fbsd-nat.c
> > > > +++ b/gdb/fbsd-nat.c
> > > > @@ -662,6 +662,9 @@ resume_one_thread_cb (struct thread_info *tp, void *data)
> > > >    if (ptid_get_pid (tp->ptid) != ptid_get_pid (*ptid))
> > > >      return 0;
> > > >
> > > > +  if (is_exited (tp->ptid))
> > > > +    return 0;
> > > > +		
> > > >    if (ptid_get_lwp (tp->ptid) == ptid_get_lwp (*ptid))
> > > >      request = PT_RESUME;
> > > >    else
> > > > @@ -680,6 +683,9 @@ resume_all_threads_cb (struct thread_info *tp, void *data)
> > > >    if (!ptid_match (tp->ptid, *filter))
> > > >      return 0;
> > > >
> > > > +  if (is_exited (tp->ptid))
> > > > +    return 0;
> > > > +		
> > > >    if (ptrace (PT_RESUME, ptid_get_lwp (tp->ptid), NULL, 0) == -1)
> > > >      perror_with_name (("ptrace"));
> > > >    return 0;
> > > > @@ -711,6 +717,7 @@ fbsd_resume (struct target_ops *ops,
> > > >    if (ptid_lwp_p (ptid))
> > > >      {
> > > >        /* If ptid is a specific LWP, suspend all other LWPs in the process.  */
> > > > +      gdb_assert (!is_exited (ptid));
> > > 
> > > If we're asserting on this (since supposedly it shouldn't happen), do we 
> > > need to check for is_exited on the two functions above?
> > > 
> > > Also, is there a reason why we're not detecting a thread that has 
> > > exited? Aren't all threads stopped at this point (for all-stop mode at 
> > > least)?
> > [...]
> > 
> > Hello,
> > 
> > I just nailed this down after it has been annoying me for some time,
> > fixed it with a similar patch as the one submitted by John, and came
> > here to report it.
> > 
> > The reason that we are "not detecting" an exited thread (at least in the
> > scenario I got is), gdb/thread.c:
> > 
> >   --- cut ---
> >   static void
> >   delete_thread_1 (ptid_t ptid, int silent)
> >   {
> >   ...
> >     /* If this is the current thread, or there's code out there that
> >        relies on it existing (refcount > 0) we can't delete yet.  Mark
> >        it as exited, and notify it.  */
> >     if (tp->refcount > 0
> >         || ptid_equal (tp->ptid, inferior_ptid))
> >       {
> >   ...
> >          /* Will be really deleted some other time.  */
> >          printf_unfiltered ("========== Will be really deleted some other time %u\n", ptid);
> >          return;
> >        }
> >   ...
> >   if (tpprev)
> >     tpprev->next = tp->next;
> >   else
> >     thread_list = tp->next;
> >   --- cut ---
> > 
> > In my scenario tp->refcount is 0, but
> > "ptid_equal (tp->ptid, inferior_ptid)" is true, so the thread's entry is
> > not removed from the global "threads_list".
> > 
> > The gdb output (with "set debug fbsd-lwp" enabled):
> > 
> >   --- cut ---
> >   FLWP: adding thread for LWP 102009
> >   [New LWP 102009 of process 40304]
> >   FLWP: fbsd_resume for ptid (-1, 0, 0)
> >   FLWP: fbsd_resume for ptid (40304, 102009, 0)
> >   FLWP: fbsd_resume for ptid (-1, 0, 0)
> >   FLWP: fbsd_resume for ptid (40304, 102009, 0)
> >   FLWP: fbsd_resume for ptid (-1, 0, 0)
> >   FLWP: deleting thread for LWP 102009
> >   [LWP 102009 of process 40304 exited]
> >   ...
> >   ptrace: No such process.
> >   --- cut ---
> > 
> > Hope this helps.
> 
> In particular, the sequence of events is this:
> 
> - an LWP (T1) reports a "normal" event (in the test case it is hitting a
>   breakpoint).  This is reported to the core and sets the current thread
>   (and thus inferior_ptid) to T1.
> - the same LWP (T1) then exits and a thread exit event is reported via
>   ptrace() to the native target.  The native target calls delete_thread,
>   but the thread is not removed, just marked EXITING since it ==
>   inferior_ptid as Vasil noted.  The native target just
>   continues the process explicitly via ptrace() without reporting any
>   event to the core aside from the call to delete_thread().
> - some other LWP (T2) reports an event (in the test case it is a
>   breakpoint).
> - the user continues which invokes fbsd_resume() which wants to resume
>   all threads.  Here iterate_over_threads() in fbsd_resume() will
>   encounters the exited thread for T1 since nothing has called
>   thread_update_list() (which would invoke delete_exited_threads() from
>   fbsd_update_thread_list()).  Since the thread is exited, trying to
>   manipulate it via ptrace() results in an error.
> 
> 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.

Luis (or anyone else), any thoughts on the above or if there is a better way
to solve this (e.g. calling delete_exited_threads() explicitly in fbsd_resume()
before iterate_threads() and/or using ALL_NONEXITED_THREADS instead of
iterate_threads)?

-- 
John Baldwin