Re: [PATCH 11/12] Use reinsert_breakpoint for vCont;s

[Yao Qi <qiyaoltc at gmail dot com>]

Pedro Alves <palves@redhat.com> writes:

> That doesn't tell the reader why we need to stop _all_ threads.  The
> threads that are about to be resumed are obviously stopped, and
> thus we could already _access_ inferior memory through them.

GDB may only resume some threads, and leave other threads running.  In
order to access inferior memory safely, we must stop all threads.

>
> I guess this is about flushing instruction caches?
>

No, it is not about flushing instruction caches.

>>>> @@ -5176,6 +5241,30 @@ proceed_all_lwps (void)
>>>>    if (debug_threads)
>>>>      debug_printf ("Proceeding, no step-over needed\n");
>>>>  
>>>> +  /* Re-install the reinsert breakpoints on software single step target
>>>> +     if the client wants it step.  */
>>>> +  if (can_software_single_step ())
>>>
>>> Not immediately obvious to why is this necessary.  Where were they
>>> removed in the first place?  I'm it must be necessary, but maybe
>>> extending the comment helps.
>> 
>> How about this
>> 
>>   /* On software single step target, we removed reinsert breakpoints
>>      after we get any events from the inferior.  
>
> Is that all events, even internal events?  From the patch, it seemed
> like it was only before reporting an event to gdb.
>

You are right, I though too much about supporting range-stepping.
I rewrite the comments in the patch below,

>> If the client wants
>>      thread step, re-install these reinsert breakpoints.  */
>> 
>
> If we only remove before reporting an event to gdb, then I don't
> understand this.  We already insert single-step breakpoints when
> we process the resume request from gdb, no?

We insert single-step breakpoints when we process the resume requests
and threads are about to be resumed.  If threads still have pending
status, single-step breakpoints are not installed, so we need to install
them in proceed_all_lwps.

-- 
Yao (éå)
From f7237f92a32f0ab98ad5db91833f85c36eaf25a3 Mon Sep 17 00:00:00 2001
From: Yao Qi <yao.qi@linaro.org>
Date: Thu, 19 May 2016 17:07:49 +0100
Subject: [PATCH] Use reinsert_breakpoint for vCont;s

V2: fix spaces in changelog entry,
    use maybe_hw_step,
    cancel step-over if signal arrives (!maybe_internal_trap),

This patch is to teach GDBserver using software single step to handle
vCont;s.  Simply speaking, if the thread's resume request is resume_step,
install reinsert breakpoint at the next pcs when GDBserver is about to
resume threads.  These reinsert breakpoints of a thread are removed,
when GDBserver gets an event from that thread.  Note that GDBserver may
or may not report this event back to GDB.

gdb/gdbserver:

2016-05-20  Yao Qi  <yao.qi@linaro.org>

	* linux-low.c (resume_stopped_resumed_lwps): If resume request
	is resume_step, call maybe_hw_step.
	(linux_wait_1): Stop all threads, remove reinsert breakpoints,
	and unstop them.
	(linux_resume_one_thread): If resume request is resume_step,
	call maybe_hw_step.
	(linux_resume): Install reinsert breakpoints if the thread is
	requested to resume_step.
	(proceed_one_lwp): If resume request is resume_step, call
	maybe_hw_step.
	(proceed_all_lwps): Install reinsert breakpoints if the thread is
	requested to resume_step.

diff --git a/gdb/gdbserver/linux-low.c b/gdb/gdbserver/linux-low.c
index 5d0eafc..dc28c0d 100644
--- a/gdb/gdbserver/linux-low.c
+++ b/gdb/gdbserver/linux-low.c
@@ -2567,7 +2567,10 @@ resume_stopped_resumed_lwps (struct inferior_list_entry *entry)
       && !lp->status_pending_p
       && thread->last_status.kind == TARGET_WAITKIND_IGNORE)
     {
-      int step = thread->last_resume_kind == resume_step;
+      int step = 0;
+
+      if (thread->last_resume_kind == resume_step)
+	step = maybe_hw_step (thread);
 
       if (debug_threads)
 	debug_printf ("RSRL: resuming stopped-resumed LWP %s at %s: step=%d\n",
@@ -3518,6 +3521,23 @@ linux_wait_1 (ptid_t ptid,
       return ignore_event (ourstatus);
     }
 
+  /* Remove reinsert breakpoints ...  */
+  if (can_software_single_step ()
+      && has_reinsert_breakpoints (current_thread)
+      /*... if GDB requests this thread doing resume_step or ...*/
+      && (current_thread->last_resume_kind == resume_step
+	  /* GDBserver has already started the step-over for vCont;s,
+	     but it gets some other signal, like SIGSTOP sent by
+	     GDBserver for vCont;t or other signal program received.  */
+	  || !maybe_internal_trap))
+    {
+      stop_all_lwps (1, event_child);
+
+      delete_reinsert_breakpoints (current_thread);
+
+      unstop_all_lwps (1, event_child);
+    }
+
   /* Note that all addresses are always "out of the step range" when
      there's no range to begin with.  */
   in_step_range = lwp_in_step_range (event_child);
@@ -4281,12 +4301,6 @@ linux_resume_one_lwp_throw (struct lwp_info *lwp,
 
       step = maybe_hw_step (thread);
     }
-  else
-    {
-      /* If the thread isn't doing step-over, there shouldn't be any
-	 reinsert breakpoints.  */
-      gdb_assert (!has_reinsert_breakpoints (thread));
-    }
 
   if (fast_tp_collecting == 1)
     {
@@ -4841,7 +4855,6 @@ linux_resume_one_thread (struct inferior_list_entry *entry, void *arg)
 {
   struct thread_info *thread = (struct thread_info *) entry;
   struct lwp_info *lwp = get_thread_lwp (thread);
-  int step;
   int leave_all_stopped = * (int *) arg;
   int leave_pending;
 
@@ -4910,10 +4923,14 @@ linux_resume_one_thread (struct inferior_list_entry *entry, void *arg)
 
   if (!leave_pending)
     {
+      int step = 0;
+
       if (debug_threads)
 	debug_printf ("resuming LWP %ld\n", lwpid_of (thread));
 
-      step = (lwp->resume->kind == resume_step);
+      if (lwp->resume->kind == resume_step)
+	step = maybe_hw_step (thread);
+
       linux_resume_one_lwp (lwp, step, lwp->resume->sig, NULL);
     }
   else
@@ -4954,6 +4971,7 @@ linux_resume (struct thread_resume *resume_info, size_t n)
   struct thread_info *need_step_over = NULL;
   int any_pending;
   int leave_all_stopped;
+  int resume_step_is_handled = 0;
 
   if (debug_threads)
     {
@@ -4997,12 +5015,55 @@ linux_resume (struct thread_resume *resume_info, size_t n)
 	debug_printf ("Resuming, no pending status or step over needed\n");
     }
 
+  /* If resume_step is requested by GDB, install reinsert breakpoints
+     when the thread is about to be actually resumed.  IOW, we don't
+     insert reinsert breakpoints if any thread has pending status.  */
+  if (!leave_all_stopped && can_software_single_step ())
+    {
+      struct inferior_list_entry *inf, *tmp;
+
+      if (debug_threads)
+	debug_printf ("Handle resume_step.\n");
+
+      ALL_INFERIORS (&all_threads, inf, tmp)
+	{
+	  struct thread_info *thread = (struct thread_info *) inf;
+	  struct lwp_info *lwp = get_thread_lwp (thread);
+
+	  if (lwp->resume != NULL && lwp->resume->kind == resume_step)
+	    {
+	      if (!resume_step_is_handled)
+		{
+		  /* We need to access the inferior memory to install
+		     reinsert breakpoints, so stop all threads.  */
+		  stop_all_lwps (0, NULL);
+
+		  if (debug_threads)
+		    debug_printf ("Done stopping all threads.\n");
+
+		  resume_step_is_handled = 1;
+		}
+
+	      install_software_single_step_breakpoints (lwp);
+
+	      if (debug_threads)
+		debug_printf ("Insert breakpoint for resume_step LWP %ld\n",
+			      lwpid_of (thread));
+	    }
+	}
+
+      if (debug_threads)
+	debug_printf ("Handle resume_step.  Done\n");
+    }
+
   /* Even if we're leaving threads stopped, queue all signals we'd
      otherwise deliver.  */
   find_inferior (&all_threads, linux_resume_one_thread, &leave_all_stopped);
 
   if (need_step_over)
     start_step_over (get_thread_lwp (need_step_over));
+  else if (resume_step_is_handled)
+    unstop_all_lwps (0, NULL);
 
   if (debug_threads)
     {
@@ -5098,7 +5159,8 @@ proceed_one_lwp (struct inferior_list_entry *entry, void *except)
       if (debug_threads)
 	debug_printf ("   stepping LWP %ld, client wants it stepping\n",
 		      lwpid_of (thread));
-      step = 1;
+
+      step = maybe_hw_step (thread);
     }
   else if (lwp->bp_reinsert != 0)
     {
@@ -5164,6 +5226,33 @@ proceed_all_lwps (void)
   if (debug_threads)
     debug_printf ("Proceeding, no step-over needed\n");
 
+  if (can_software_single_step ())
+    {
+      struct inferior_list_entry *inf, *tmp;
+
+      ALL_INFERIORS (&all_threads, inf, tmp)
+	{
+	  struct thread_info *thread = (struct thread_info *) inf;
+
+	  /* On software single step target, we insert reinsert
+	     breakpoints when the threads are about to be actually
+	     resumed.  IOW, we don't insert them if any thread has
+	     pending status.  Before we proceed threads, insert
+	     reinsert breakpoints if the client wants it step.  */
+	  if (thread->last_resume_kind == resume_step)
+	    {
+	      struct lwp_info *lwp = get_thread_lwp (thread);
+
+	      if (!has_reinsert_breakpoints (thread))
+		install_software_single_step_breakpoints (lwp);
+
+	      if (debug_threads)
+		debug_printf ("Insert breakpoint for resume_step LWP %ld\n",
+			      lwpid_of (thread));
+	    }
+	}
+    }
+
   find_inferior (&all_threads, proceed_one_lwp, NULL);
 }