Re: [PATCH 2/2] Avoid step-over infinite loop in GDBServer

[Antoine Tremblay <antoine dot tremblay at ericsson dot com>]

Pedro Alves writes:

> On 11/29/2016 12:07 PM, Antoine Tremblay wrote:
>>  - GDBServer stops on instruction A in thread 1.
>>  - Deletes thread 1 single-step breakpoint.
>>  - Starts a step-over of thread 1 to step-over the thread 2 breakpoint.
>>  - GDBServer finishes a step-over and is at instruction B.
>>  - GDBserver starts a step-over of thread 1 to step-over the thread 3
>>    breakpoint at instruction B.
>>  - GDBServer stops on instuction A in thread 1.
>>  - GDBServer is now in an infinite loop.
>
> This sounds to me very much like a fairness issue.  There were
> three threads stopped that needed to move past a breakpoint, but
> gdbserver always picks thread 1.  Why?

It is a fairness issue but not between the stepping over threads,
gdbserver could pick any thread rather than thread 1 and still get into
the same loop.

The problem is one of fairness between stepping over threads and
non-stepping-over threads.

For example if all the instructions in the thread's execution
path have a breakpoint on it such that this thread always needs to
step-over and that the only way to break that loop is to let a thread
that doesn't need a step-over run then we're in an infinite loop since
GDBServer always gives precedence to the threads needing a step-over.

Thus this patch created more fairness by giving non-stepping over
threads a chance to run.

But it doesn't work... I'm not sure this can be fixed it might be more
a problem like putting a breakpoint on a mutex and wanting a thread that
depends on that mutex to continue... so more a user problem... but
non-stop-fair-events is done in a way that this can happen...

Ideas are welcome :)