[lldb-dev] "step" threading issues

[jingham at apple.com]

On May 2, 2013, at 10:20 AM, Carlo Kok <ck at remobjects.com> wrote:

> Op 2-5-2013 18:47, jingham at apple.com schreef:
>> 
>> On May 2, 2013, at 3:41 AM, Carlo Kok <ck at remobjects.com> wrote:
>> 
>>> Op 2-5-2013 12:03, Carlo Kok schreef:
>>>> Op 29-4-2013 22:15, Carlo Kok schreef:
>>>>> Op 29-4-2013 18:41, Carlo Kok schreef:
>>>>>> Op 29-4-2013 18:23, Greg Clayton schreef:
>>>>>>> You should only get one stopped event unless you are hitting a
>>>>>>> breakpoint that continues your target. In this case the eStateStopped
>>>>>>> event would be a "restarted" event which can be found out by:
>>>>>>> 
>>>>>>>     static bool
>>>>>>>     SBProcess::GetRestartedFromEvent (const lldb::SBEvent &event);
>>>>>>> 
>>>>>>> This means the program stopped but restarted automatically. You should
>>>>>>> never see two eStateStopped events in a row, if you are, please try
>>>>>>> and reproduce on a Mac target and file a bug.
>>>>>> 
>>>>>> Indeed that's my problem. I get several of those with reason "stop" for
>>>>>> StepInto. I'm up to date on last weeks trunk update to windows; but I'll
>>>>>> try to compile lldb on OSX to see if I can reproduce it there.
>>>>> 
>>>>> 
>>>>> I get this from the log:
>>>>> http://pastebin.com/msnqdi6P
>>>>> 
>>>>> at line 1656 it resumes it, yet still broadcast a "stop", which makes no
>>>>> sense to me, nor can I find any way this could happen, I do know it
>>>>> doesn't happen if i slowly step through it.
>>>> 
>>>> I've narrowed it down to this (line 1622):
>>>> 
>>>> ThreadList::ShouldReportStop 3 threads
>>>> Thread::ShouldReportStop() tid = 0x1a03: returning vote  for complete
>>>> stack's back plan
>>>> ...
>>>> ThreadList::ShouldReportStop returning yes
>>>> 
>>>> ShouldReportStop returns "Yes" because
>>>> m_completed_plan_stack.count > 0 in which case it returns:
>>>> return m_completed_plan_stack.back()->ShouldReportStop (event_ptr);
>>>> 
>>>> Now the last thing in that is a:
>>>> Vote
>>>> ThreadPlanCallFunction::ShouldReportStop(Event *event_ptr)
>>>> {
>>>>     if (m_takedown_done || IsPlanComplete())
>>>>         return eVoteYes; << goes here.
>>>>     else
>>>>         return ThreadPlan::ShouldReportStop(event_ptr);
>>>> }
>>>> 
>>>> Which is the call to g_lookup_implementation_no_stret_function_code.
>>>> 
>>>> Does anyone have an idea what else I can check to solve this step into
>>>> issue?
>>>> 
>>>> I've not been able to reproduce this on Osx or Linux.
>>> 
>>> If i change:
>>> if (m_completed_plan_stack.size() > 0)
>>> to:
>>> if (m_completed_plan_stack.size() > 0 && m_plan_stack.size() == 0)
>>> 
>>> in Thread::ShouldReportStop, it works perfectly.
>> 
>> Yes, but that's because then this branch will never get called (m_plan_stack.size() is never 0, there's always a base plan.
>> 
>> So this isn't a correct fix.
> 
> I figured it wouldn't be that simple. However it cannot be right that it a: resumes the process  and b: returns "yes let the public api know we stopped" at the same time.

I disagree.  You need that for instance to implement "process handle SOMESIG --stop false --print true".  You are auto-continuing, yet you want to tell the event-loop runner that this happened so that it can notify about it however is appropriate.  For instance in the case of the lldb driver we listen to this event and print some bit to the console.  But a GUI might want to do this in some different way, so I don't want to just dump something to stdout and hope somebody notices...  

Also we send an event if a breakpoint condition or command is hit but continues the process so that a UI would know to update hit counts in its breakpoint display.  

The stopped event always says it restarted (you can query this with the Process::ProcessEventData::GetRestartedFromEvent API.)  You just have to make sure you check that any time you get a stopped event.

I have to fix the ThreadPlan.h docs to be more clear about how ShouldReportStop works, however (and I should change its name to be a little more explicit.)  ShouldReportStop only gets called if the process is going to auto-continue after the stop.  That makes sense, I can't see why you would want to have the process really stop and NOT tell the agent running the event loop about it.  But it isn't clear from the name.  Whoever did ThreadPlanCallFunction probably didn't realize this, since it shouldn't be returning true from ShouldReportStop.  After all, if some thread plan ran a function and decided on the basis of the results of that function call to auto-continue, then there's no reason to tell the outside world about that.  I'd have to think a little more carefully to be 100% sure that there aren't any cases where this would be useful, but I can't think of any right now.

OTOH, it looks like the Linux port of LLDB is for some reason not resilient to these "auto-continue" events.  That puzzles me, since this should all be handled in generic execution control logic, and this sort of thing causes no problems on OS X.

Jim

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