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

[Antoine Tremblay <antoine.tremblay@ericsson.com>]

Ping.

Antoine Tremblay writes:

> Before this patch, GDBServer always executed a step-over if it found a
> thread that needed one.
>
> This could be a problem in a situation exposed by non-stop-fair-events.exp
> where the code and the breakpoint placement is like so:
>
> instruction A : has a single-step breakpoint installed for thread 1 and 2
> instruction B : has a single-step breakpoint installed for thread 3
> and is a branch to A.
>
> In this particular case:
>
>  - 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 patch avoids this issue by counting the number of times a thread does
> a step-over consecutively.  If the thread reaches a threshold, which is
> currently 32, GDBServer will not step-over but rather restart all the
> threads.
>
> I chose a threshold of 32, so to trigger this there needs to be 32
> consecutive instructions with breakpoints installed that one thread needs
> to step over. I think this should be rare enought to trigger only in this
> case but suggestions on the threshold value are welcome.
>
> If the threshold is hit and the step-over is delayed, when the thread
> that needed a step-over runs it will simply hit the breakpoint it needed
> to step-over and retry to start a step-over.
>
> This makes it possible for other threads to run and start a step-over
> dance of their own or pending events like signals to be handled.
>
> This patch fixes the intermittent FAILs for
> gdb.threads/non-stop-fair-events.exp on ARM like discussed here:
> https://sourceware.org/ml/gdb-patches/2016-11/msg00132.html
>
> No regressions, tested on ubuntu 14.04 ARMv7.
> With gdbserver-native/-m{arm,thumb}
>
> gdb/gdbserver/ChangeLog:
>
> 	* linux-low.c (class step_over_limiter): New class.
> 	(_step_over_limiter): New global variable.
> 	(linux_wait_1): Count step-overs.
> 	(proceed_all_lwps): Delay step-over if threshold is reached.
> ---
>  gdb/gdbserver/linux-low.c | 76 +++++++++++++++++++++++++++++++++++++++++++----
>  1 file changed, 70 insertions(+), 6 deletions(-)
>
> diff --git a/gdb/gdbserver/linux-low.c b/gdb/gdbserver/linux-low.c
> index 15fb726..b84b62a 100644
> --- a/gdb/gdbserver/linux-low.c
> +++ b/gdb/gdbserver/linux-low.c
> @@ -282,6 +282,53 @@ static int proceed_one_lwp (struct inferior_list_entry *entry, void *except);
>     being stepped.  */
>  ptid_t step_over_bkpt;
>  
> +/* Class limiting the number of consecutive step-overs for a thread.  */
> +
> +class step_over_limiter
> +{
> + public:
> +
> +  step_over_limiter () : m_ptid (null_ptid), m_count (0), m_max_count (32) {}
> +
> +  void step_over_done (struct lwp_info *lwp)
> +  {
> +    ptid_t ptid = lwp->thread->entry.id;
> +
> +    if (!ptid_equal (ptid, m_ptid))
> +      {
> +	m_ptid = ptid;
> +	m_count = 0;
> +      }
> +
> +    m_count++;
> +  }
> +
> +  bool can_step_over (struct lwp_info *lwp)
> +  {
> +    if (!ptid_equal(lwp->thread->entry.id, m_ptid)
> +	|| m_count < m_max_count)
> +      return true;
> +    else
> +      {
> +	/* Reset here so that the step_over is retried.  */
> +	m_ptid = null_ptid;
> +	m_count = 0;
> +	return false;
> +      }
> +  }
> +
> + private:
> +
> +  ptid_t m_ptid;
> +  int m_count;
> +
> +  /* Maximum step overs for a thread, before all threads are run instead of
> +     stepping over.  */
> +  const int m_max_count;
> +};
> +
> +step_over_limiter _step_over_limiter;
> +
>  /* True if the low target can hardware single-step.  */
>  
>  static int
> @@ -3607,6 +3654,8 @@ linux_wait_1 (ptid_t ptid,
>  	     doesn't lose it.  */
>  	  enqueue_pending_signal (event_child, WSTOPSIG (w), info_p);
>  
> +	  _step_over_limiter.step_over_done (event_child);
> +
>  	  proceed_all_lwps ();
>  	}
>        else
> @@ -3694,6 +3743,8 @@ linux_wait_1 (ptid_t ptid,
>  	     We're going to keep waiting, so use proceed, which
>  	     handles stepping over the next breakpoint.  */
>  	  unsuspend_all_lwps (event_child);
> +
> +	  _step_over_limiter.step_over_done (event_child);
>  	}
>        else
>  	{
> @@ -5400,13 +5451,26 @@ proceed_all_lwps (void)
>  
>        if (need_step_over != NULL)
>  	{
> -	  if (debug_threads)
> -	    debug_printf ("proceed_all_lwps: found "
> -			  "thread %ld needing a step-over\n",
> -			  lwpid_of (need_step_over));
> +	  /* Don't step over if we're looping too much.  */
> +	  if (_step_over_limiter.can_step_over
> +	      (get_thread_lwp (need_step_over)))
> +	    {
> +	      if (debug_threads)
> +		debug_printf ("proceed_all_lwps: found "
> +			      "thread %ld needing a step-over\n",
> +			      lwpid_of (need_step_over));
>  
> -	  start_step_over (get_thread_lwp (need_step_over));
> -	  return;
> +	      start_step_over (get_thread_lwp (need_step_over));
> +	      return;
> +	    }
> +	  else
> +	    {
> +	      if (debug_threads)
> +		debug_printf ("proceed_all_lwps: found "
> +			      "thread %ld needing a step-over "
> +			      "but max consecutive step-overs reached\n",
> +			      lwpid_of (need_step_over));
> +	    }
>  	}
>      }