Questions on commit 6c95b8df7fef5273da71c34775918c554aae0ea8

Questions on commit 6c95b8df7fef5273da71c34775918c554aae0ea8

[Doug Evans]

[Ugh, bad To address on first try.]

Hi.

While looking into removing lwp_list from gdb (akin to what I did for
gdbserver) I found that several bits of target code are calling
init_thread_list
(grep init_thread_list *.c).
Maybe there's the odd case where target code would need to do this,
but normally when should target code *ever* do this?

[
side notes:
- having to deal with target code doing common operations (for lack of
a better phrase) is something I have to fix in another context
(prologue skipping), so it's now a bit of a pet peeve of mine
- it's likely there are more such cases (e.g. init_wait_for_inferior),
but for the nonce I'm picking init_thread_list as a basis for
discussion
- these issues are not specific to or introduced by the commit
specified in the subject line, it's just a concrete base from which to
phrase my questions
]

To try to assist you with getting a handle on my confusion, consider
remote.c:extended_remote_create_inferior_1 from gdb 6.8:

  /* Clean up from the last time we were running.  */
  init_thread_list ();
  init_wait_for_inferior ();

Now look at what's there today:

  if (!have_inferiors ())
    {
      /* Clean up from the last time we ran, before we mark the target
         running again.  This will mark breakpoints uninserted, and
         get_offsets may insert breakpoints.  */
      init_thread_list ();
      init_wait_for_inferior ();
    }

I think(!) there may be multiple ways to look at all of this as being
wrong, so pick your favorite, but here's one way: What does it matter
whether there are other inferiors as to whether
remote.c:extended_remote_create_inferior has to "clean up from the
last time we were running"?

Obviously we can't call init_thread_list if there are other inferiors,
but why are we calling init_thread_list here at all?  Why isn't this
state being managed by gdb itself (inf*.c or some such)?  I can
imagine one can look at this as just being still a work in progress on
the way to proper multi-target support.  It's stil a bit odd though to
have taken this step this way, so I'm hoping for some clarification.

Another related question I have is: Why does remote-sim.c:gdbsim_create_inferior
call init_wait_for_inferior unconditionally whereas the above code
conditions the call on !have_inferiors()?  Maybe it's a simple
oversight, but I think (emphasis on THINK, I could certainly be
missing something) we need to take a step back and ask why this code
is there at all.  Putting this code in target routines gives us a lot
of flexibility, but the cost is more mental load (for lack of a better
phrase) and more touch points when trying to fix/improve core gdb, and
I'm getting the impression that the pendulum has swung too far towards
putting general housekeeping operations in target code.

Re: Questions on commit 6c95b8df7fef5273da71c34775918c554aae0ea8

[Pedro Alves]

On 09/20/2014 07:39 PM, Doug Evans wrote:
> [Ugh, bad To address on first try.]
> 
> Hi.
> 
> While looking into removing lwp_list from gdb (akin to what I did for
> gdbserver) I found that several bits of target code are calling
> init_thread_list
> (grep init_thread_list *.c).
> Maybe there's the odd case where target code would need to do this,
> but normally when should target code *ever* do this?

> To try to assist you with getting a handle on my confusion, consider
> remote.c:extended_remote_create_inferior_1 from gdb 6.8:
> 
>   /* Clean up from the last time we were running.  */
>   init_thread_list ();
>   init_wait_for_inferior ();
> 
> Now look at what's there today:
> 
>   if (!have_inferiors ())
>     {
>       /* Clean up from the last time we ran, before we mark the target
>          running again.  This will mark breakpoints uninserted, and
>          get_offsets may insert breakpoints.  */
>       init_thread_list ();
>       init_wait_for_inferior ();
>     }

> 
> I think(!) there may be multiple ways to look at all of this as being
> wrong, so pick your favorite, but here's one way: What does it matter
> whether there are other inferiors as to whether
> remote.c:extended_remote_create_inferior has to "clean up from the
> last time we were running"?
> 
> Obviously we can't call init_thread_list if there are other inferiors,
> but why are we calling init_thread_list here at all?  Why isn't this
> state being managed by gdb itself (inf*.c or some such)?  I can
> imagine one can look at this as just being still a work in progress on
> the way to proper multi-target support.  It's stil a bit odd though to
> have taken this step this way, so I'm hoping for some clarification.

Really not sure what sort of answer you're looking for.

I don't really recall history on that level of detail,
but, before 6c95b8df, the only kind of multi-process GDB supported,
was the model used by DICOS.  On that platform, we didn't use
"run" at all, only "attach" and all programs shared the program
space.  Well, before 6c95b8df, GDB didn't even have the
concept of a program space, but it didn't matter; even now that
it does, a single program space is the model that best fits.

So when we got to making GDB's multi-process fit for Linux,
which was the point of that patch, we must have stumbled on that
init_thread_list call in remote.c:extended_remote_create_inferior_1,
as soon as we did two "run"s in a row.  So the change in that
patch just looks like the conservative change.

The comment change you show wasn't done by that patch.
A trivial git blame 6c95b8df^ points at:

commit 45280a5259f209ba74ed8255674a3fd345307a55
Author:     Daniel Jacobowitz <drow@false.org>
AuthorDate: Thu May 8 16:08:10 2008 +0000

        * remote.c (extended_remote_create_inferior_1): Clean up
        before marking the target running.

I didn't dig deeper than that, but git blame further will
probably point out something ancient.

> Another related question I have is: Why does remote-sim.c:gdbsim_create_inferior
> call init_wait_for_inferior unconditionally whereas the above code
> conditions the call on !have_inferiors()? 

Most probably because nobody has ever tried making remote sim work
with multiple processes at the same time, so nobody ever stumbled
on that.

> Maybe it's a simple
> oversight, but I think (emphasis on THINK, I could certainly be
> missing something) we need to take a step back and ask why this code
> is there at all.  Putting this code in target routines gives us a lot
> of flexibility, but the cost is more mental load (for lack of a better
> phrase) and more touch points when trying to fix/improve core gdb, and
> I'm getting the impression that the pendulum has swung too far towards
> putting general housekeeping operations in target code.

Huh?  I think you're getting this backwards.  You make it sound like
we've been adding more of this stuff in target code ("putting"),
while instead these are _ancient_ code that over the years we've
been cleaning up.

Why not just git blame on that one?  It shows:

40b92220 (Jim Kingdon          1993-09-17 17:27:43 +0000  467)
8501c742 (Stu Grossman         1996-08-13 00:01:37 +0000  468)   gdbsim_kill ();
40b92220 (Jim Kingdon          1993-09-17 17:27:43 +0000  469)   remove_breakpoints ();
ec25d19b (Steve Chamberlain    1993-01-03 22:36:04 +0000  470)   init_wait_for_inferior ();
ec25d19b (Steve Chamberlain    1993-01-03 22:36:04 +0000  471)

Thanks,
Pedro Alves

Re: Questions on commit 6c95b8df7fef5273da71c34775918c554aae0ea8

[Doug Evans]

Pedro Alves writes:
 > On 09/20/2014 07:39 PM, Doug Evans wrote:
 > > [Ugh, bad To address on first try.]
 > > 
 > > Hi.
 > > 
 > > While looking into removing lwp_list from gdb (akin to what I did for
 > > gdbserver) I found that several bits of target code are calling
 > > init_thread_list
 > > (grep init_thread_list *.c).
 > > Maybe there's the odd case where target code would need to do this,
 > > but normally when should target code *ever* do this?
 > 
 > > To try to assist you with getting a handle on my confusion, consider
 > > remote.c:extended_remote_create_inferior_1 from gdb 6.8:
 > > 
 > >   /* Clean up from the last time we were running.  */
 > >   init_thread_list ();
 > >   init_wait_for_inferior ();
 > > 
 > > Now look at what's there today:
 > > 
 > >   if (!have_inferiors ())
 > >     {
 > >       /* Clean up from the last time we ran, before we mark the target
 > >          running again.  This will mark breakpoints uninserted, and
 > >          get_offsets may insert breakpoints.  */
 > >       init_thread_list ();
 > >       init_wait_for_inferior ();
 > >     }
 > 
 > > 
 > > I think(!) there may be multiple ways to look at all of this as being
 > > wrong, so pick your favorite, but here's one way: What does it matter
 > > whether there are other inferiors as to whether
 > > remote.c:extended_remote_create_inferior has to "clean up from the
 > > last time we were running"?
 > > 
 > > Obviously we can't call init_thread_list if there are other inferiors,
 > > but why are we calling init_thread_list here at all?  Why isn't this
 > > state being managed by gdb itself (inf*.c or some such)?  I can
 > > imagine one can look at this as just being still a work in progress on
 > > the way to proper multi-target support.  It's stil a bit odd though to
 > > have taken this step this way, so I'm hoping for some clarification.
 > 
 > Really not sure what sort of answer you're looking for.

The most succinct way of expressing what I'm looking for that I can
think of is that I want to understand this code, and I don't.
There are several instances of the !have_inferiors() check, I picked
this one because it seemed like as good a choice as any.

 > I don't really recall history on that level of detail,
 > but, before 6c95b8df, the only kind of multi-process GDB supported,
 > was the model used by DICOS.  On that platform, we didn't use
 > "run" at all, only "attach" and all programs shared the program
 > space.  Well, before 6c95b8df, GDB didn't even have the
 > concept of a program space, but it didn't matter; even now that
 > it does, a single program space is the model that best fits.
 > 
 > So when we got to making GDB's multi-process fit for Linux,
 > which was the point of that patch, we must have stumbled on that
 > init_thread_list call in remote.c:extended_remote_create_inferior_1,
 > as soon as we did two "run"s in a row.  So the change in that
 > patch just looks like the conservative change.

If this is just a temp hack to play it safe, then that explains it.
["this" being the wrapping of the calls to init_thread_list
and init_wait_for_inferior with "if (!have_inferiors ())"]
I read what I could from the thread when the patch was submitted
and I couldn't find anything. I don't know this code well enough
to be comfortable with any assumptions.

It's too bad FIXMEs are frowned on in gdb-land. I think this is a good
site for one.  Assuming this is just a work-in-progress, the code really
is confusing. For example, what if have_inferiors() returns TRUE?
That means that we won't "Clean up from the last time we ran, ...".
The reader is left with questions like
"Then what? Is the system left in a broken state because
we haven't cleaned up? Or is the cleanup done elsewhere? If so, where?
Or do things just not need to be cleaned up in this case? If so, why?"

OTOH, if a FIXME isn't appropriate here, and the code is ok as-is
(I'm not making a value judgement here, I still have to understand the code
first), I'd still like to understand it so I can add a comment to help the next
reader (which if it's me a year from now I may well have forgotten this
conversation :-)).

 > The comment change you show wasn't done by that patch.
 > A trivial git blame 6c95b8df^ points at:
 > 
 > commit 45280a5259f209ba74ed8255674a3fd345307a55
 > Author:     Daniel Jacobowitz <drow@false.org>
 > AuthorDate: Thu May 8 16:08:10 2008 +0000
 > 
 >         * remote.c (extended_remote_create_inferior_1): Clean up
 >         before marking the target running.
 > 
 > I didn't dig deeper than that, but git blame further will
 > probably point out something ancient.

Yeah, I went back a bit in the history, but it didn't help me
understand the code that's there today.

 > > Another related question I have is: Why does remote-sim.c:gdbsim_create_inferior
 > > call init_wait_for_inferior unconditionally whereas the above code
 > > conditions the call on !have_inferiors()? 
 > 
 > Most probably because nobody has ever tried making remote sim work
 > with multiple processes at the same time, so nobody ever stumbled
 > on that.
 >
 > > Maybe it's a simple
 > > oversight, but I think (emphasis on THINK, I could certainly be
 > > missing something) we need to take a step back and ask why this code
 > > is there at all.  Putting this code in target routines gives us a lot
 > > of flexibility, but the cost is more mental load (for lack of a better
 > > phrase) and more touch points when trying to fix/improve core gdb, and
 > > I'm getting the impression that the pendulum has swung too far towards
 > > putting general housekeeping operations in target code.
 > 
 > Huh?  I think you're getting this backwards.  You make it sound like
 > we've been adding more of this stuff in target code ("putting"),
 > while instead these are _ancient_ code that over the years we've
 > been cleaning up.

Well, yes we have been adding non-target-specific code to target files.
E.g., I don't see how the trustability of compilers to emit good
data for prologues varies by target, and yet we're doing different things
in amd64_skip_prologue vs arm_skip_prologue w.r.t. gcc vs clang.
I could certainly be wrong here, but I don't think so.
Can gcc get this right for ARM enough that we accept the risk of the
odd bug whereas with AMD64 we've chosen to not accept this risk?
Here's a case where some cleanup is needed, but this is relatively
freshly added code.
[From one perspective *-tdep files are different than
target files like remote.c. But it's still a case of balancing
target-specific and non-target-specific code.]

Plus, pending an understanding of the !have_inferiors() check above (and
elsewhere), it's not clear to me we have always been cleaning up. I can't
know until I understand why things are the way they are. And if we haven't
always been cleaning up maybe more awareness is needed. OTOH, maybe we have,
so I'm asking.

So in addition to understanding the !have_inferiors() checks,
I want to bring this issue up to help raise awareness.
Not with anyone in particular but in general.
I only put you and Stan in the To line because I
think you guys can shed light on the use of !have_inferiors().

 > Why not just git blame on that one?  It shows:
 > 
 > 40b92220 (Jim Kingdon          1993-09-17 17:27:43 +0000  467)
 > 8501c742 (Stu Grossman         1996-08-13 00:01:37 +0000  468)   gdbsim_kill ();
 > 40b92220 (Jim Kingdon          1993-09-17 17:27:43 +0000  469)   remove_breakpoints ();
 > ec25d19b (Steve Chamberlain    1993-01-03 22:36:04 +0000  470)   init_wait_for_inferior ();
 > ec25d19b (Steve Chamberlain    1993-01-03 22:36:04 +0000  471)

Yeah. Alas this data didn't help with my question.

Re: Questions on commit 6c95b8df7fef5273da71c34775918c554aae0ea8

[Pedro Alves]

On 09/30/2014 08:50 PM, Doug Evans wrote:
> Pedro Alves writes:
>  > On 09/20/2014 07:39 PM, Doug Evans wrote:
>  > > [Ugh, bad To address on first try.]
>  > > 
>  > > Hi.
>  > > 
>  > > While looking into removing lwp_list from gdb (akin to what I did for
>  > > gdbserver) I found that several bits of target code are calling
>  > > init_thread_list
>  > > (grep init_thread_list *.c).
>  > > Maybe there's the odd case where target code would need to do this,
>  > > but normally when should target code *ever* do this?
>  > 
>  > > To try to assist you with getting a handle on my confusion, consider
>  > > remote.c:extended_remote_create_inferior_1 from gdb 6.8:
>  > > 
>  > >   /* Clean up from the last time we were running.  */
>  > >   init_thread_list ();
>  > >   init_wait_for_inferior ();
>  > > 
>  > > Now look at what's there today:
>  > > 
>  > >   if (!have_inferiors ())
>  > >     {
>  > >       /* Clean up from the last time we ran, before we mark the target
>  > >          running again.  This will mark breakpoints uninserted, and
>  > >          get_offsets may insert breakpoints.  */
>  > >       init_thread_list ();
>  > >       init_wait_for_inferior ();
>  > >     }
>  > 
>  > > 
>  > > I think(!) there may be multiple ways to look at all of this as being
>  > > wrong, so pick your favorite, but here's one way: What does it matter
>  > > whether there are other inferiors as to whether
>  > > remote.c:extended_remote_create_inferior has to "clean up from the
>  > > last time we were running"?
>  > > 
>  > > Obviously we can't call init_thread_list if there are other inferiors,
>  > > but why are we calling init_thread_list here at all?  Why isn't this
>  > > state being managed by gdb itself (inf*.c or some such)?  I can
>  > > imagine one can look at this as just being still a work in progress on
>  > > the way to proper multi-target support.  It's stil a bit odd though to
>  > > have taken this step this way, so I'm hoping for some clarification.
>  > 
>  > Really not sure what sort of answer you're looking for.
> 
> The most succinct way of expressing what I'm looking for that I can
> think of is that I want to understand this code, and I don't.
> There are several instances of the !have_inferiors() check, I picked
> this one because it seemed like as good a choice as any.

init_thread_list does two things:

 - wipes all threads.
 - resets the thread id counter back to 0, so the next
   thread is gdb thread number 1.

The main point of calling init_thread_list if there are
no live inferiors, is to reset the thread id counter.
That is so that with "run; kill; run", you end up with
the main thread being gdb thread number 1, in both
runs.

Now, if there are other live inferiors, then we shouldn't wipe
their threads.  And as gdb thread ids are global currently,
not private per-inferior/process, then we obviously shouldn't
reset the  thread id counter either.

The reason this isn't managed by the core itself, is I believe
that nobody has tried to do that yet.

>  > > Maybe it's a simple
>  > > oversight, but I think (emphasis on THINK, I could certainly be
>  > > missing something) we need to take a step back and ask why this code
>  > > is there at all.  Putting this code in target routines gives us a lot
>  > > of flexibility, but the cost is more mental load (for lack of a better
>  > > phrase) and more touch points when trying to fix/improve core gdb, and
>  > > I'm getting the impression that the pendulum has swung too far towards
>  > > putting general housekeeping operations in target code.
>  > 
>  > Huh?  I think you're getting this backwards.  You make it sound like
>  > we've been adding more of this stuff in target code ("putting"),
>  > while instead these are _ancient_ code that over the years we've
>  > been cleaning up.
> 
> Well, yes we have been adding non-target-specific code to target files.
> E.g., I don't see how the trustability of compilers to emit good
> data for prologues varies by target, and yet we're doing different things
> in amd64_skip_prologue vs arm_skip_prologue w.r.t. gcc vs clang.

Well, on the compiler side, prologue emission
is tied to the backend.  Certainly it's easy to picture that different
backends on the compiler side vary in quality.

For example, I recall that on GCC,
there was a long term conversion of prologue emission from text
to rtl.  A quick google search finds:

 https://gcc.gnu.org/ml/gcc/2011-06/msg00222.html
and
 https://gcc.gnu.org/ml/gcc-patches/2011-07/msg00685.html

for example.

I don't know whether something like that affects the cases
in question or not.  I'm just saying that I could see there
being a reason.

> I could certainly be wrong here, but I don't think so.
> Can gcc get this right for ARM enough that we accept the risk of the
> odd bug whereas with AMD64 we've chosen to not accept this risk?
> Here's a case where some cleanup is needed, but this is relatively
> freshly added code.
> [From one perspective *-tdep files are different than
> target files like remote.c. But it's still a case of balancing
> target-specific and non-target-specific code.]
> 
> Plus, pending an understanding of the !have_inferiors() check above (and
> elsewhere), it's not clear to me we have always been cleaning up. 

I honestly fail to see the relation and I don't think it's useful
to try to generalize like that.  Each case is a different case.

If you find patterns that can be factored out, and find ways to
clean things up, then please go for it.

Thanks,
Pedro Alves

Re: Questions on commit 6c95b8df7fef5273da71c34775918c554aae0ea8

[Doug Evans]

Pedro Alves <palves@redhat.com> writes:
>> The most succinct way of expressing what I'm looking for that I can
>> think of is that I want to understand this code, and I don't.
>> There are several instances of the !have_inferiors() check, I picked
>> this one because it seemed like as good a choice as any.
>
> init_thread_list does two things:
>
>  - wipes all threads.
>  - resets the thread id counter back to 0, so the next
>    thread is gdb thread number 1.
>
> The main point of calling init_thread_list if there are
> no live inferiors, is to reset the thread id counter.
> That is so that with "run; kill; run", you end up with
> the main thread being gdb thread number 1, in both
> runs.

That part I can understand.

> Now, if there are other live inferiors, then we shouldn't wipe
> their threads.  And as gdb thread ids are global currently,
> not private per-inferior/process, then we obviously shouldn't
> reset the  thread id counter either.

This I can grok too.

However, what if some code is not properly clearing a thread
from the list? (which may involve just tagging it as exited and leaving
gc'ing it until later)  Such a bug will be hidden in the
!have_inferiors() case because we take a sledgehammer to the list.
To the reader it's not clear whether wiping the list is necessary,
or just follows along for the ride, so to speak, and is essentially
a no-op, because we're using init_thread_list to reset thread numbering.

I would expect the attached patch to be a no-op.
I don't have an opinion on committing it.
I do have an opinion that the current code is confusing,
and this is one way to help clear it up.

2014-10-19  Doug Evans  <xdje42@gmail.com>

	* thread.c (reset_thread_numbering): New function.
	(init_thread_list): Call it.
	* gdbthread.h (reset_thread_numbering): Declare.
	* fork-child.c (fork_inferior): Call reset_thread_numbering instead of
	init_thread_list.
	* infcmd.c (kill_command): Ditto.
	(detach_command): Ditto.
	* remote.c (extended_remote_create_inferior): Ditto.

diff --git a/gdb/fork-child.c b/gdb/fork-child.c
index 4c316b1..149eb22 100644
--- a/gdb/fork-child.c
+++ b/gdb/fork-child.c
@@ -379,7 +379,10 @@ fork_inferior (char *exec_file_arg, char *allargs, char **env,
   environ = save_our_env;
 
   if (!have_inferiors ())
-    init_thread_list ();
+    {
+      /* Reset thread numbering to begin back at 1.  */
+      reset_thread_numbering ();
+    }
 
   inf = current_inferior ();
 
diff --git a/gdb/gdbthread.h b/gdb/gdbthread.h
index fb47bae..4b3bf7f 100644
--- a/gdb/gdbthread.h
+++ b/gdb/gdbthread.h
@@ -261,6 +261,11 @@ struct thread_info
   struct btrace_thread_info btrace;
 };
 
+/* Reset thread numbering so that they begin at 1 again.
+   This can only be called when it is known that there are no current
+   non-exited threads.  A typical test is !have_inferiors().  */
+extern void reset_thread_numbering (void);
+
 /* Create an empty thread list, or empty the existing one.  */
 extern void init_thread_list (void);
 
diff --git a/gdb/infcmd.c b/gdb/infcmd.c
index 4415b31..0d06996 100644
--- a/gdb/infcmd.c
+++ b/gdb/infcmd.c
@@ -2399,7 +2399,8 @@ kill_command (char *arg, int from_tty)
      with their threads.  */
   if (!have_inferiors ())
     {
-      init_thread_list ();		/* Destroy thread info.  */
+      /* Reset thread numbering to begin back at 1.  */
+      reset_thread_numbering ();
 
       /* Killing off the inferior can leave us with a core file.  If
 	 so, print the state we are left in.  */
@@ -2787,10 +2788,11 @@ detach_command (char *args, int from_tty)
   if (!gdbarch_has_global_solist (target_gdbarch ()))
     no_shared_libraries (NULL, from_tty);
 
-  /* If we still have inferiors to debug, then don't mess with their
-     threads.  */
   if (!have_inferiors ())
-    init_thread_list ();
+    {
+      /* No more inferiors, reset thread numbering back to 1.  */
+      reset_thread_numbering ();
+    }
 
   if (deprecated_detach_hook)
     deprecated_detach_hook ();
diff --git a/gdb/remote.c b/gdb/remote.c
index 20f2988..6c43da8 100644
--- a/gdb/remote.c
+++ b/gdb/remote.c
@@ -8035,10 +8035,12 @@ extended_remote_create_inferior (struct target_ops *ops,
 
   if (!have_inferiors ())
     {
+      /* Reset thread numbering to begin back at 1.  */
+      reset_thread_numbering ();
+
       /* Clean up from the last time we ran, before we mark the target
 	 running again.  This will mark breakpoints uninserted, and
 	 get_offsets may insert breakpoints.  */
-      init_thread_list ();
       init_wait_for_inferior ();
     }
 
diff --git a/gdb/thread.c b/gdb/thread.c
index 5c94264..789ab7e 100644
--- a/gdb/thread.c
+++ b/gdb/thread.c
@@ -195,15 +195,30 @@ free_thread (struct thread_info *tp)
   xfree (tp);
 }
 
+/* See gdbthread.h.  */
+
 void
-init_thread_list (void)
+reset_thread_numbering (void)
 {
-  struct thread_info *tp, *tpnext;
+  struct thread_info *tp;
+
+  /* While IWBN to assert thread_list is NULL, there could still be
+     threads on it (e.g., if you detach from the current inferior).
+     Instead verify all threads on the list have exited.  They will be
+     garbage-collected as we're able to.  */
+  for (tp = thread_list; tp != NULL; tp = tp->next)
+    gdb_assert (tp->state == THREAD_EXITED);
 
   highest_thread_num = 0;
 
-  if (!thread_list)
-    return;
+  /* Take the opportunity to update this since we know it's zero.  */
+  threads_executing = 0;
+}
+
+void
+init_thread_list (void)
+{
+  struct thread_info *tp, *tpnext;
 
   for (tp = thread_list; tp; tp = tpnext)
     {
@@ -212,7 +227,8 @@ init_thread_list (void)
     }
 
   thread_list = NULL;
-  threads_executing = 0;
+
+  reset_thread_numbering ();
 }
 
 /* Allocate a new thread with target id PTID and add it to the thread

Re: Questions on commit 6c95b8df7fef5273da71c34775918c554aae0ea8

[Pedro Alves]

On 10/19/2014 11:56 PM, Doug Evans wrote:

> However, what if some code is not properly clearing a thread
> from the list? (which may involve just tagging it as exited and leaving
> gc'ing it until later)  Such a bug will be hidden in the
> !have_inferiors() case because we take a sledgehammer to the list.
> To the reader it's not clear whether wiping the list is necessary,
> or just follows along for the ride, so to speak, and is essentially
> a no-op, because we're using init_thread_list to reset thread numbering.
> 
> I would expect the attached patch to be a no-op.
> I don't have an opinion on committing it.
> I do have an opinion that the current code is confusing,
> and this is one way to help clear it up.
> 
> 2014-10-19  Doug Evans  <xdje42@gmail.com>
> 
> 	* thread.c (reset_thread_numbering): New function.
> 	(init_thread_list): Call it.
> 	* gdbthread.h (reset_thread_numbering): Declare.
> 	* fork-child.c (fork_inferior): Call reset_thread_numbering instead of
> 	init_thread_list.
> 	* infcmd.c (kill_command): Ditto.
> 	(detach_command): Ditto.
> 	* remote.c (extended_remote_create_inferior): Ditto.
> 
> diff --git a/gdb/fork-child.c b/gdb/fork-child.c
> index 4c316b1..149eb22 100644
> --- a/gdb/fork-child.c
> +++ b/gdb/fork-child.c
> @@ -379,7 +379,10 @@ fork_inferior (char *exec_file_arg, char *allargs, char **env,
>    environ = save_our_env;
>  
>    if (!have_inferiors ())
> -    init_thread_list ();
> +    {
> +      /* Reset thread numbering to begin back at 1.  */
> +      reset_thread_numbering ();
> +    }
>  
>    inf = current_inferior ();
>  
> diff --git a/gdb/gdbthread.h b/gdb/gdbthread.h
> index fb47bae..4b3bf7f 100644
> --- a/gdb/gdbthread.h
> +++ b/gdb/gdbthread.h
> @@ -261,6 +261,11 @@ struct thread_info
>    struct btrace_thread_info btrace;
>  };
>  
> +/* Reset thread numbering so that they begin at 1 again.
> +   This can only be called when it is known that there are no current
> +   non-exited threads.  A typical test is !have_inferiors().  */
> +extern void reset_thread_numbering (void);
> +
>  /* Create an empty thread list, or empty the existing one.  */
>  extern void init_thread_list (void);
>  
> diff --git a/gdb/infcmd.c b/gdb/infcmd.c
> index 4415b31..0d06996 100644
> --- a/gdb/infcmd.c
> +++ b/gdb/infcmd.c
> @@ -2399,7 +2399,8 @@ kill_command (char *arg, int from_tty)
>       with their threads.  */
>    if (!have_inferiors ())
>      {
> -      init_thread_list ();		/* Destroy thread info.  */
> +      /* Reset thread numbering to begin back at 1.  */
> +      reset_thread_numbering ();
>  
>        /* Killing off the inferior can leave us with a core file.  If
>  	 so, print the state we are left in.  */
> @@ -2787,10 +2788,11 @@ detach_command (char *args, int from_tty)
>    if (!gdbarch_has_global_solist (target_gdbarch ()))
>      no_shared_libraries (NULL, from_tty);
>  
> -  /* If we still have inferiors to debug, then don't mess with their
> -     threads.  */
>    if (!have_inferiors ())
> -    init_thread_list ();
> +    {
> +      /* No more inferiors, reset thread numbering back to 1.  */
> +      reset_thread_numbering ();
> +    }
>  
>    if (deprecated_detach_hook)
>      deprecated_detach_hook ();
> diff --git a/gdb/remote.c b/gdb/remote.c
> index 20f2988..6c43da8 100644
> --- a/gdb/remote.c
> +++ b/gdb/remote.c
> @@ -8035,10 +8035,12 @@ extended_remote_create_inferior (struct target_ops *ops,
>  
>    if (!have_inferiors ())
>      {
> +      /* Reset thread numbering to begin back at 1.  */
> +      reset_thread_numbering ();

IMO this function name is clear enough that we wouldn't
need to add that same comment to all callers.

> +
>        /* Clean up from the last time we ran, before we mark the target
>  	 running again.  This will mark breakpoints uninserted, and
>  	 get_offsets may insert breakpoints.  */
> -      init_thread_list ();
>        init_wait_for_inferior ();
>      }
>  
> diff --git a/gdb/thread.c b/gdb/thread.c
> index 5c94264..789ab7e 100644
> --- a/gdb/thread.c
> +++ b/gdb/thread.c
> @@ -195,15 +195,30 @@ free_thread (struct thread_info *tp)
>    xfree (tp);
>  }
>  
> +/* See gdbthread.h.  */
> +
>  void
> -init_thread_list (void)
> +reset_thread_numbering (void)
>  {
> -  struct thread_info *tp, *tpnext;
> +  struct thread_info *tp;
> +
> +  /* While IWBN to assert thread_list is NULL, there could still be
> +     threads on it (e.g., if you detach from the current inferior).

Not sure I understand this detach reference.  Are you seeing exited threads
after a detach from the current inferior?  What are the steps you used?
detach_inferior and exit_inferior are called with inferior_ptid
pointing at null_ptid so that thread can be deleted.

> +     Instead verify all threads on the list have exited.  They will be
> +     garbage-collected as we're able to.  */
> +  for (tp = thread_list; tp != NULL; tp = tp->next)
> +    gdb_assert (tp->state == THREAD_EXITED);

Hmm, this doesn't make that much sense to me.  Either we have an
empty list, or we can't reset the thread numbers.  Otherwise, the
next thread we add might reuse the GDB ID of one of those
exited threads, if it also reuses the target side ID (which
can easily happen when the target fakes thread ids, e.g.,
remote.c:magic_null_ptid).

Thinking about this further, I think the cases that may legitimately leave
an exited thread in the list are related to thread's reference count
having been incremented, either because a make_cleanup_restore_current_thread
cleanup is in the cleanup chain, or the user did "thread apply all
some-command-that-wipes-threads" -- detach/kill/etc., but also could
also be the remote connection closing abruptly while running any command.

do_restore_current_thread_cleanup won't restore the dead thread
as selected thread, but I think the thread will still be
in the list.

This then suggest to me that what we need is a function that
tries to garbage collect exited threads, and then call
that without checking have_inferiors() at specific points
We could start with the current places we call init_thread_list,
and look for somewhere more central in a following step.
prune_threads is close, but I don't think there's need to
query anything off of the target.

Resetting the thread numbering would still have to only be
done under have_inferiors(), or, perhaps even clearer,
only if the thread list is empty after the purging.
IOW, say that the thread numbering always starts at 1 if we're
adding a thread to an empty thread list.  With this in mind, it
might be simpler to instead reset to the thread numbering back to
1 in new_thread.

WDYT?

Thanks,
Pedro Alves