[lldb-dev] Continuing from a breakpoint with multiple threads

Adrian McCarthy amccarth at google.com
Mon Jun 1 08:04:53 PDT 2015

Thanks for the info.

This is not theoretical.  I'm trying to get TestBreakAfterJoin to pass on
Windows.  Step 1 was to convert it use <thread> instead of <pthreads.h>.
Step 2 was to fix some minor issues in TargetThreadWindows.

But now the inferior deadlocks because the one thread that's not suspended
is waiting on the ones that are.  Once the ThreadPlanStepOverBreakpoint
plan is popped, the current plan is ThreadPlanBase, which, as far as I can
tell, does nothing to resume suspended threads.

I'll compare this to what happens on another platform to see if there
should be some other thread plan in use.


On Fri, May 29, 2015 at 4:50 PM, Jim Ingham <jingham at apple.com> wrote:

> When stepping over a breakpoint, the ThreadPlanStepOverBreakpoint gets
> pushed, handles the single instruction step - during which it does suspend
> the other threads - then it gets popped.  When you next resume, whatever
> plan was handling the stepping before the breakpoint was hit will resume
> with whatever policy for running other threads it was using.
> So it's up to the plan that was on the stack before the
> "StepOverBreakpoint" was pushed to decide this.  Most of the more complex
> plans (like step-over/step-into) try to keep the other threads from running
> if possible (unless the user instructed otherwise) but they also will let
> all the threads run if there's something going on that might deadlock.  For
> instance, if you are doing "next" and we step through straight-line
> instructions in a function, we will only run the one thread you are
> stepping in (by default, you can control this with options to the "thread
> step-over" command.   But if we step into a function, we set a breakpoint
> on the return address and then run with all threads resumed because
> stepping out of a function could run arbitrary code.
> Anyway, was this a theoretical question, or do you have some instance
> where you are actually seeing a deadlock?
> Jim
> > On May 29, 2015, at 1:59 PM, Adrian McCarthy <amccarth at google.com>
> wrote:
> >
> > [I'm trying to make TestBreakAfterJoin work on Windows.]
> >
> > I'm unclear how continuing from a breakpoint in a multi-threaded
> inferior is supposed to work.
> >
> > A breakpoint is set, and the inferior runs until one of its threads hits
> the breakpoint.  The user then selects continue.
> >
> > The thread that had hit the breakpoint has a thread plan type of
> ThreadPlanStepOverBreakpoint, which causes all of the other threads to be
> set to state eStateSuspended.  The thread that had hit the breakpoint then
> steps beyond the breakpoint, and the breakpoint is restored.  The thread is
> then resumed again.
> >
> > But the other threads are all still suspended, causing the inferior to
> deadlock.
> >
> > The question is:  Where should the other threads have their resume
> states set back to a running state?
> >
> > Adrian
> > _______________________________________________
> > lldb-dev mailing list
> > lldb-dev at cs.uiuc.edu
> > http://lists.cs.uiuc.edu/mailman/listinfo/lldb-dev
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