[lldb-dev] Process plugin implementation with multiple threads

jingham at apple.com jingham at apple.com
Fri Nov 14 15:26:45 PST 2014

Is there a way to ask Windows "give me all the threads for this process"?  If so, the path of least resistance would be to ignore the Thread Created events in this loop, and then only generate the thread list on demand when someone has stopped.

At some point we'll want to do keep alive debugging and then we'll have to handle posting interesting events while the process is still running.  But I don't want to do that piecemeal.

For now, putting threads in the thread list while the debugger is running from both the private & public state's perspective does no good, and putting them in between a stop & an immediate continue doesn't do anybody any good either.  You're just making your plugin more fragile.


> On Nov 14, 2014, at 3:08 PM, Zachary Turner <zturner at google.com> wrote:
> The way Windows debug notifications work is like this:
> while (WaitForDebugEvent(&event))   // Block until something happens in the process
> {
>    // Windows stopped the process for us automatically.  Do something with the event
>    ContinueDebugEvent(&event);  // Resume the process
> }
> After WaitForDebugEvent and before I call ContinueDebugEvent, the event might say that a new thread was created.  So at this point I'd like to update the thread list.  The process is actually stopped here, but it shouldn't appear so to the user. So at the beginning I tell LLDB the process is stopped -- SetPrivateState(eStateStopped), and at the end I tell it that it's running again -- SetPrivateState(eStateRunning).
> However, this code is still running on a different from what LLDB is going to invoke methods on the plugin.  RefreshStateAfterStop, DoResume, etc are all going to be running on a different thread from this loop.  The code is simpler if I can just stick the new thread in the ThreadList (or whatever other state needs updating if it's not thread related) right here from this thread.  Alternatively, when I set the state to eStateStopped presumably that will trigger LLDB to call RefreshStateAfterStop() from the other thread.  I can also modify the state then, it just requiers a little extra work as I have to do some synchronization and marshalling / storage of the event parameters.
> On Fri Nov 14 2014 at 2:53:35 PM Greg Clayton <gclayton at apple.com> wrote:
> > On Nov 14, 2014, at 10:15 AM, Zachary Turner <zturner at google.com> wrote:
> >
> > Windows doesn't need to do anything special for each thread as it is created. Only need to update the thread list in the Process object as far as we're concerned, but that's it.
> >
> > The main thing I'm worried about is that the Process object itself runs a couple of threads in the background (the private state thread, for example), so I was worried about possible race conditions when modifying state asynchronously (i.e. from my debugger thread).
> What state needs to be updated asynchronously? You shouldn't be touching the thread list (no need to update it, you should be able to update it on demand each time you actually stop right?) at all until you have a valid stop reason and then the process will update the threads itself when and if it needs them.
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