[lldb-dev] Process plugin implementation with multiple threads

Zachary Turner zturner at google.com
Fri Nov 14 15:46:55 PST 2014

It's possible, but Windows gives us a handle to the thread which has all
privileges associated with it (in other words we can use it for just about
anything), so it's considered better to use the handle windows gives us. At
the very least we'd want to queue up all the incoming event notifications
that we get from this loop, and then use it to update the state when
someone stops.

Just to be certain, when you and Greg refer to "when someone has stopped",
I assume you are referring to a public stop, i.e. someone has to enter a
command to resume the debugger?

I was wondering why my RefreshStateAfterStop override was never being
called, I'm guessing now that it's because I was only updating the private
state, and this only gets called on a public stop.

On Fri Nov 14 2014 at 3:26:47 PM <jingham at apple.com> wrote:

> 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.
> Jim
> > 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.
> > _______________________________________________
> > lldb-dev mailing list
> > lldb-dev at cs.uiuc.edu
> > http://lists.cs.uiuc.edu/mailman/listinfo/lldb-dev
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