[lldb-dev] Limbo

Thu Dec 20 10:57:36 PST 2012

On Dec 20, 2012, at 10:30 AM, "Kaylor, Andrew" <andrew.kaylor at intel.com> wrote:

> Like Matt, I have no idea why eStopReasonTrace was chosen here.  It probably stems from the fact that the limbo state happens in response to a "PTRACE" event, so probably guessed PTRACE_EVENT==eStopReasonTrace.

Ah, interesting.  The single-step bit on a processor is sometimes called the "trace" bit, so eStopReasonTrace was the stop reason you get when you set the trace bit and allow the process to run, and then it comes back having executed a single instruction.  Instruction single stepping is used a lot to implement the various stepping strategies, so calling a stop eStopReasonTrace when it is really just some random stop or other is definitely going to confuse us.  I'm sure Greg and I thought that naming was as self-evident as eStopReasonBreakpoint.  But we should document the stop reasons to avoid future confusion.

> 
> Frankly, I'm not even clear what the purpose of the limbo state is.  We get there because the Linux ProcessMonitor::Launch method calls PTRACE(...PTRACE_O_TRACEEXIT...), and a comment there says, "This is used to keep the child in limbo until it is destroyed."  So it seems like someone thought we should be stopping there.  (FWIW, svn blame attributes the comment to 'wilsons' as part of the original Linux process plugin on 7/23/2010.)  I notice that the PTRACE_O_TRACEEXIT call is currently excluded for FreeBSD.
> 
> The ptrace man page says this about PTRACE_O_TRACEEXIT:
> 
> "This stop will be done early during process exit when registers are still available, allowing the tracer to see where the exit occurred, whereas the normal exit notification is done after the process is finished exiting. Even though context is available, the tracer cannot prevent the exit from happening at this point."
> 
> So it might be a useful place to let the user stop and look around, but I wouldn't think we'd ordinarily want to stop there.

Right.  There's a similar thing with shared library loads.  For the most part you would never want to stop there, that would just annoy you.  But for some odd cases you might want to.  Similarly for when a program spawns a subprogram, makes a thread, etc.  Right now there's no way to do that from the lldb command line, but I want add that possibility for all these sorts of program events.

Jim

> 
> -Andy
> 
> -----Original Message-----
> From: Jim Ingham [mailto:jingham at apple.com] 
> Sent: Thursday, December 20, 2012 10:21 AM
> To: Kopec, Matt
> Cc: Kaylor, Andrew; Greg Clayton; lldb-dev at cs.uiuc.edu
> Subject: Re: [lldb-dev] Limbo
> 
> Though there's probably some busy-work with handling a new stop reason, that would be my vote.  If you make it eStopReasonNone, it will be hard to get us to do something non-trivial with the stop should we choose to.  At some point, I want to expand the "target stop-hook" mechanism so you can hook into a bunch of interesting system events, including shared library loads, process spawning and thread creation and destruction.  So while we're at it it probably is worth putting in something for this thread exit.
> 
> Jim
> 
> 
> On Dec 20, 2012, at 8:40 AM, "Kopec, Matt" <matt.kopec at intel.com> wrote:
> 
>> I'm unsure why it is eStopReasonTrace, I think that's something we inherited. Would eStopReasonNone or a new thread exiting stop reason be a better candidate?
>> ________________________________________
>> From: Jim Ingham [jingham at apple.com]
>> Sent: Wednesday, December 19, 2012 2:12 PM
>> To: Kaylor, Andrew
>> Cc: Greg Clayton; Kopec, Matt; lldb-dev at cs.uiuc.edu
>> Subject: Re: [lldb-dev] Limbo
>> 
>> Why is the stop reason for hitting this POSIXLimboStopInfo eStopReasonTrace.  That's specifically the stop reason for single stepping, which should generally be handled by the plan that was doing the single stepping (and thus why there didn't need to be a handler for it in the Base thread plan.)  I have no objections to adding a handler in ThreadPlanBase, but it seems weird to me that that's the stop reason for this Thread Exit stop event.
>> 
>> Jim
>> 
>> On Dec 19, 2012, at 10:47 AM, "Kaylor, Andrew" <andrew.kaylor at intel.com> wrote:
>> 
>>> The current trunk implementation of POSIXLimboStopInfo is returning 'true' for both ShouldStop() and ShouldNotify().  Having ShouldStop() return 'false' gets the process to run to completion, but I get a line saying "Process <pid> stopped and was programmatically restarted" even if I also return 'false' from ShouldNotify().
>>> 
>>> To get rid of the 'restarted' message, I also have to add an 'eStopReasonTrace' handler to ThreadPlanBase.
>>> 
>>> The attached patch addresses all three of these changes.  If it looks right to everyone else I'll commit it.  (BTW, this is adapted from some earlier work that Matt did that had never been committed to trunk, but I think we've done some testing with it here on Linux, Mac and FreeBSD.)
>>> 
>>> -Andy
>>> 
>>> -----Original Message-----
>>> From: Greg Clayton [mailto:gclayton at apple.com]
>>> Sent: Tuesday, December 18, 2012 4:14 PM
>>> To: Kopec, Matt
>>> Cc: Kaylor, Andrew; Jim Ingham; lldb-dev at cs.uiuc.edu
>>> Subject: Re: [lldb-dev] Limbo
>>> 
>>> 
>>> On Dec 18, 2012, at 2:34 PM, "Kopec, Matt" <matt.kopec at intel.com> wrote:
>>> 
>>>> When multi-threading debugging works on Linux, this signal would be received for any inferior thread which exits, including non-main spawned threads. It would be possible for another thread to hit a breakpoint in this case. I'm wondering whether its' even useful to stop lldb/create a limbo stop reason when a thread exits? Is there any usefulness to examining a thread in limbo state (ie. a thread finished execution, it's about to exit. we can read registers...)? if anything, we would update the process thread list to remove the exiting thread and make sure it exits but I don't think the debugger needs to stop for this.
>>> 
>>> If the thread is exiting and nothing can be done with it, it shouldn't even be in the process thread list. Just omit any threads in this state and do any cleanup needed to reap it/let it die.
>>> 
>>>> 
>>>> Matt
>>>> ________________________________________
>>>> From: lldb-dev-bounces at cs.uiuc.edu [lldb-dev-bounces at cs.uiuc.edu] on behalf of Kaylor, Andrew [andrew.kaylor at intel.com]
>>>> Sent: Tuesday, December 18, 2012 5:12 PM
>>>> To: Jim Ingham
>>>> Cc: lldb-dev at cs.uiuc.edu
>>>> Subject: Re: [lldb-dev] Limbo
>>>> 
>>>> Hi Jim,
>>>> 
>>>> We're setting the limbo state because we got a 'SIGTRAP | PTRACE_EVENT_EXIT << 8' signal -- that is, the inferior process is exiting.  It looks like this is only getting used by Linux and FreeBSD.
>>>> 
>>>> I'm not sure it's even possible for another thread to hit a breakpoint at this stage, but if it is then the behavior you describe is what we'd want.
>>>> 
>>>> -Andy
>>>> 
>>>> -----Original Message-----
>>>> From: Jim Ingham [mailto:jingham at apple.com]
>>>> Sent: Tuesday, December 18, 2012 1:51 PM
>>>> To: Kaylor, Andrew
>>>> Cc: lldb-dev at cs.uiuc.edu
>>>> Subject: Re: [lldb-dev] Limbo
>>>> 
>>>> I don't know enough about the Linux threading model to know what is really going on.  Is the thread being in this "Limbo" state the reason why the process as a whole stopped, or did it stop for some other reason, and the thread in Limbo is just along for the ride?  If the latter, then should it have a stop reason at all?  In general, in lldb, threads only have stop reasons if they were one of the threads that caused the process to stop.
>>>> 
>>>> You are achieving pretty much the same thing by returning false from its ShouldStop.  But note that if you happen to hit a breakpoint on another thread when the Limbo'ed thread exists, then both threads will be reported to have stopped, one with reason breakpoint and one "thread exited".  Is that what you want?
>>>> 
>>>> Jim
>>>> 
>>>> On Dec 18, 2012, at 1:24 PM, "Kaylor, Andrew" <andrew.kaylor at intel.com> wrote:
>>>> 
>>>>> There's an issue on Linux where LLDB stop with "stop reason = thread exited" and displays a brief assembly dump from somewhere in libc.  This seems to happen because it is stopping in the "limbo" state.  I can make it go away by having POSIXLimboStopInfo::ShouldStop() return false instead of true.
>>>>> 
>>>>> Is there any reason I shouldn't do that?
>>>>> 
>>>>> Thanks,
>>>>> Andy
>>>>> _______________________________________________
>>>>> lldb-dev mailing list
>>>>> lldb-dev at cs.uiuc.edu
>>>>> http://lists.cs.uiuc.edu/mailman/listinfo/lldb-dev
>>>> 
>>>> 
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>>> 
>>> <limbo-stop-plan.patch>
>> 
> 