Re: [PATCH 01/16 v2] Refactor native follow-fork

Breazeal, Don
2014-09-05T20:20:08Z

One clarification...

On 9/5/2014 11:56 AM, Breazeal, Don wrote:
> Hi Pedro,
> Thanks for reviewing this.
>
> On 9/5/2014 7:20 AM, Pedro Alves wrote:
>> linux_child_follow_fork ends up with:
>>
>> static int
>> linux_child_follow_fork (struct target_ops *ops, int follow_child,
>> int detach_fork)
>> {
>> int has_vforked;
>> int parent_pid, child_pid;
>>
>> has_vforked = (inferior_thread ()->pending_follow.kind
>> == TARGET_WAITKIND_VFORKED);
>> parent_pid = ptid_get_lwp (inferior_ptid);
>> ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
>> if (parent_pid == 0)
>> parent_pid = ptid_get_pid (inferior_ptid);
>> child_pid
>> = ptid_get_pid (inferior_thread ()->pending_follow.value.related_pid);
>>
>> if (!follow_child)
>> {
>> ...
>> }
>> else
>> {
>> struct lwp_info *child_lp;
>>
>> child_lp = add_lwp (inferior_ptid);
>> ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
>> child_lp->stopped = 1;
>> child_lp->last_resume_kind = resume_stop;
>>
>> /* Let the thread_db layer learn about this new process. */
>> check_for_thread_db ();
>> }
>> }
>>
>> Nothing appears to switch inferior_ptid to the child, so seems
>> like we're adding the child_lp with the wrong lwp (and calling
>> check_for_thread_db in the wrong context) ? Is this managing
>> to work by chance because follow_fork_inferior leaves inferior_ptid
>> pointing to the child?
>
> Yes, follow_fork_inferior always sets inferior_ptid to the followed
> inferior. On entry, linux_child_follow_fork expects inferior_ptid to be
> the followed inferior. So I think it is getting the correct inferior
> from inferior_ptid in these cases. I can change that if you prefer; see
> my question below about acceptable solutions.

Er... I can change how inferior_ptid is passed to
linux_child_follow_fork, not whether the correct ptid is used. :-P

>
> Regarding check_for_thread_db, there is something unrelated that I don't
> understand here. If we have reached this function, then aren't we
> guaranteed that PTRACE_O_TRACECLONE is supported, and that we are using
> that instead of libthread_db for detecting thread events? If so, why do
> we need to call check_for_thread_db at all?
>
> Then this at the top uses the wrong
>> inferior_thread ():
>>
>> has_vforked = (inferior_thread ()->pending_follow.kind
>> == TARGET_WAITKIND_VFORKED);
>>
>>
>> and we're lucky that nothing end up using has_vforked in the
>> follow child path?
>
> You are right, this is incorrect and unnecessary in the case where we
> are following the child.
>
>>
>> I'd much rather we don't have these assumptions in place.
>
> Would an acceptable solution be to move the definitions and assignments
> of has_vforked, parent_pid, and child_pid into the follow-parent case,
> as below?
>
> Would you also prefer that on entry to linux_child_follow_fork,
> inferior_ptid is set to the parent like it was before, or would a
> comment explaining that inferior_ptid is expected to be the followed
> inferior be sufficient?
>
> static int
> linux_child_follow_fork (struct target_ops *ops, int follow_child,
> int detach_fork)
> {
> if (!follow_child)
> {
> struct lwp_info *child_lp = NULL;
> int status = W_STOPCODE (0);
> struct cleanup *old_chain;
> int has_vforked;
> int parent_pid, child_pid;
>
> has_vforked = (inferior_thread ()->pending_follow.kind
> == TARGET_WAITKIND_VFORKED);
> parent_pid = ptid_get_lwp (inferior_ptid);
> if (parent_pid == 0)
> parent_pid = ptid_get_pid (inferior_ptid);
> child_pid
> = ptid_get_pid (inferior_thread
> ()->pending_follow.value.related_pid);
>
>>
>> These files / targets also have to_follow_fork implementations:
>>
>> inf-ptrace.c: t->to_follow_fork = inf_ptrace_follow_fork;
>> inf-ttrace.c: t->to_follow_fork = inf_ttrace_follow_fork;
>>
>> which will break if we don't adjust them as well. Did you
>> check whether the refactored code (follow_fork_inferior)
>> makes sense for those?
>
> I completely missed these; sorry about that. In theory I should be able
> to make similar changes to these that maintains the existing
> functionality. I don't currently have a way (that I know of) to test
> either of them. Testing requires a non-Linux version of Unix and an
> HP-UX system, correct? I'll start work on the changes in spite of that.
>
>>
>> Thanks,
>> Pedro Alves
>>
>
> Thanks,
> --Don
>
>