Embed the pending step-over chain in thread_info objects

In order to teach non-stop mode to do in-line step-overs (pause all
threads, remove breakpoint, single-step, reinsert breakpoint, restart
threads), we'll need to be able to queue in-line step over requests,
much like we queue displaced stepping (out-of-line) requests.
Actually, the queue should be the same -- threads wait for their turn
to step past something (breakpoint, watchpoint), doesn't matter what
technique we end up using when the step over actually starts.

I found that the queue management ends up simpler and more efficient
if embedded in the thread objects themselves.  This commit converts
the existing displaced stepping queue to that.  Later patches will
make the in-line step-overs code paths use it too.

gdb/ChangeLog:
2015-08-07  Pedro Alves  <palves@redhat.com>

	* gdbthread.h (struct thread_info) <step_over_prev,
	step_over_next>: New fields.
	(thread_step_over_chain_enqueue, thread_step_over_chain_remove)
	(thread_step_over_chain_next, thread_is_in_step_over_chain): New
	declarations.
	* infrun.c (struct displaced_step_request): Delete.
	(struct displaced_step_inferior_state) <step_request_queue>:
	Delete field.
	(displaced_step_prepare): Assert that trap_expected is set.  Use
	thread_step_over_chain_enqueue.  Split starting a new displaced
	step to ...
	(start_step_over): ... this new function.
	(resume): Assert the thread isn't waiting for a step over already.
	(proceed): Assert the thread isn't waiting for a step over
	already.
	(infrun_thread_stop_requested): Adjust to remove threads from the
	embedded step-over chain.
	(handle_inferior_event) <fork/vfork>: Call start_step_over after
	displaced_step_fixup.
	(handle_signal_stop): Call start_step_over after
	displaced_step_fixup.
	* infrun.h (step_over_queue_head): New declaration.
	* thread.c (step_over_chain_enqueue, step_over_chain_remove)
	(thread_step_over_chain_next, thread_is_in_step_over_chain)
	(thread_step_over_chain_enqueue)
	(thread_step_over_chain_remove): New functions.
	(delete_thread_1): Remove thread from the step-over chain.

1 files changed, 66 insertions, 69 deletions

diff --git a/gdb/infrun.c b/gdb/infrun.c
index d3cd0ae..9cd897a 100644
--- a/gdb/infrun.c
+++ b/gdb/infrun.c
@@ -1221,6 +1221,15 @@ follow_exec (ptid_t ptid, char *execd_pathname)
      matically get reset there in the new process.).  */
 }
 
+/* The queue of threads that need to do a step-over operation to get
+   past e.g., a breakpoint.  What technique is used to step over the
+   breakpoint/watchpoint does not matter -- all threads end up in the
+   same queue, to maintain rough temporal order of execution, in order
+   to avoid starvation, otherwise, we could e.g., find ourselves
+   constantly stepping the same couple threads past their breakpoints
+   over and over, if the single-step finish fast enough.  */
+struct thread_info *step_over_queue_head;
+
 /* Bit flags indicating what the thread needs to step over.  */
 
 enum step_over_what
@@ -1417,12 +1426,6 @@ step_over_info_valid_p (void)
    displaced step operation on it.  See displaced_step_prepare and
    displaced_step_fixup for details.  */
 
-struct displaced_step_request
-{
-  ptid_t ptid;
-  struct displaced_step_request *next;
-};
-
 /* Per-inferior displaced stepping state.  */
 struct displaced_step_inferior_state
 {
@@ -1432,10 +1435,6 @@ struct displaced_step_inferior_state
   /* The process this displaced step state refers to.  */
   int pid;
 
-  /* A queue of pending displaced stepping requests.  One entry per
-     thread that needs to do a displaced step.  */
-  struct displaced_step_request *step_request_queue;
-
   /* If this is not null_ptid, this is the thread carrying out a
      displaced single-step in process PID.  This thread's state will
      require fixing up once it has completed its step.  */
@@ -1667,6 +1666,9 @@ displaced_step_prepare (ptid_t ptid)
      support displaced stepping.  */
   gdb_assert (gdbarch_displaced_step_copy_insn_p (gdbarch));
 
+  /* Nor if the thread isn't meant to step over a breakpoint.  */
+  gdb_assert (tp->control.trap_expected);
+
   /* Disable range stepping while executing in the scratch pad.  We
      want a single-step even if executing the displaced instruction in
      the scratch buffer lands within the stepping range (e.g., a
@@ -1682,28 +1684,13 @@ displaced_step_prepare (ptid_t ptid)
     {
       /* Already waiting for a displaced step to finish.  Defer this
 	 request and place in queue.  */
-      struct displaced_step_request *req, *new_req;
 
       if (debug_displaced)
 	fprintf_unfiltered (gdb_stdlog,
-			    "displaced: defering step of %s\n",
+			    "displaced: deferring step of %s\n",
 			    target_pid_to_str (ptid));
 
-      new_req = xmalloc (sizeof (*new_req));
-      new_req->ptid = ptid;
-      new_req->next = NULL;
-
-      if (displaced->step_request_queue)
-	{
-	  for (req = displaced->step_request_queue;
-	       req && req->next;
-	       req = req->next)
-	    ;
-	  req->next = new_req;
-	}
-      else
-	displaced->step_request_queue = new_req;
-
+      thread_step_over_chain_enqueue (tp);
       return 0;
     }
   else
@@ -1853,24 +1840,45 @@ displaced_step_fixup (ptid_t event_ptid, enum gdb_signal signal)
   do_cleanups (old_cleanups);
 
   displaced->step_ptid = null_ptid;
+}
 
-  /* Are there any pending displaced stepping requests?  If so, run
-     one now.  Leave the state object around, since we're likely to
-     need it again soon.  */
-  while (displaced->step_request_queue)
+/* Are there any pending step-over requests?  If so, run all we can
+   now.  */
+
+static void
+start_step_over (void)
+{
+  struct thread_info *tp, *next;
+
+  for (tp = step_over_queue_head; tp != NULL; tp = next)
     {
-      struct displaced_step_request *head;
       ptid_t ptid;
+      struct displaced_step_inferior_state *displaced;
       struct regcache *regcache;
       struct gdbarch *gdbarch;
       CORE_ADDR actual_pc;
       struct address_space *aspace;
+      struct inferior *inf = find_inferior_ptid (tp->ptid);
+
+      next = thread_step_over_chain_next (tp);
 
-      head = displaced->step_request_queue;
-      ptid = head->ptid;
-      displaced->step_request_queue = head->next;
-      xfree (head);
+      displaced = get_displaced_stepping_state (inf->pid);
 
+      /* If this inferior already has a displaced step in process,
+	 don't start a new one.  */
+      if (!ptid_equal (displaced->step_ptid, null_ptid))
+	continue;
+
+      thread_step_over_chain_remove (tp);
+
+      if (step_over_queue_head == NULL)
+	{
+	  if (debug_infrun)
+	    fprintf_unfiltered (gdb_stdlog,
+				"infrun: step-over queue now empty\n");
+	}
+
+      ptid = tp->ptid;
       context_switch (ptid);
 
       regcache = get_thread_regcache (ptid);
@@ -1905,7 +1913,6 @@ displaced_step_fixup (ptid_t event_ptid, enum gdb_signal signal)
 	    target_resume (ptid, 0, GDB_SIGNAL_0);
 
 	  /* Done, we're stepping a thread.  */
-	  break;
 	}
       else
 	{
@@ -1933,6 +1940,10 @@ displaced_step_fixup (ptid_t event_ptid, enum gdb_signal signal)
 	  /* This request was discarded.  See if there's any other
 	     thread waiting for its turn.  */
 	}
+
+      /* A new displaced stepping sequence started.  Maybe we can
+	 start a displaced step on a thread of other process.
+	 Continue looking.  */
     }
 }
 
@@ -1953,10 +1964,6 @@ infrun_thread_ptid_changed (ptid_t old_ptid, ptid_t new_ptid)
     {
       if (ptid_equal (displaced->step_ptid, old_ptid))
 	displaced->step_ptid = new_ptid;
-
-      for (it = displaced->step_request_queue; it; it = it->next)
-	if (ptid_equal (it->ptid, old_ptid))
-	  it->ptid = new_ptid;
     }
 }
 
@@ -2135,6 +2142,8 @@ resume (enum gdb_signal sig)
 
   tp->stepped_breakpoint = 0;
 
+  gdb_assert (!thread_is_in_step_over_chain (tp));
+
   QUIT;
 
   /* Depends on stepped_breakpoint.  */
@@ -2654,6 +2663,8 @@ proceed (CORE_ADDR addr, enum gdb_signal siggnal)
   /* Fill in with reasonable starting values.  */
   init_thread_stepping_state (tp);
 
+  gdb_assert (!thread_is_in_step_over_chain (tp));
+
   if (addr == (CORE_ADDR) -1)
     {
       if (pc == stop_pc
@@ -2959,35 +2970,17 @@ infrun_thread_stop_requested_callback (struct thread_info *info, void *arg)
 static void
 infrun_thread_stop_requested (ptid_t ptid)
 {
-  struct displaced_step_inferior_state *displaced;
-
-  /* PTID was requested to stop.  Remove it from the displaced
-     stepping queue, so we don't try to resume it automatically.  */
-
-  for (displaced = displaced_step_inferior_states;
-       displaced;
-       displaced = displaced->next)
-    {
-      struct displaced_step_request *it, **prev_next_p;
-
-      it = displaced->step_request_queue;
-      prev_next_p = &displaced->step_request_queue;
-      while (it)
-	{
-	  if (ptid_match (it->ptid, ptid))
-	    {
-	      *prev_next_p = it->next;
-	      it->next = NULL;
-	      xfree (it);
-	    }
-	  else
-	    {
-	      prev_next_p = &it->next;
-	    }
+  struct thread_info *tp;
 
-	  it = *prev_next_p;
-	}
-    }
+  /* PTID was requested to stop.  Remove matching threads from the
+     step-over queue, so we don't try to resume them
+     automatically.  */
+  ALL_NON_EXITED_THREADS (tp)
+    if (ptid_match (tp->ptid, ptid))
+      {
+	if (thread_is_in_step_over_chain (tp))
+	  thread_step_over_chain_remove (tp);
+      }
 
   iterate_over_threads (infrun_thread_stop_requested_callback, &ptid);
 }
@@ -4045,6 +4038,9 @@ Cannot fill $_exitsignal with the correct signal number.\n"));
 	       that this operation also cleans up the child process for vfork,
 	       because their pages are shared.  */
 	    displaced_step_fixup (ecs->ptid, GDB_SIGNAL_TRAP);
+	    /* Start a new step-over in another thread if there's one
+	       that needs it.  */
+	    start_step_over ();
 
 	    if (ecs->ws.kind == TARGET_WAITKIND_FORKED)
 	      {
@@ -4293,6 +4289,7 @@ handle_signal_stop (struct execution_control_state *ecs)
      the PC, so do it here, before we set stop_pc.)  */
   displaced_step_fixup (ecs->ptid,
 			ecs->event_thread->suspend.stop_signal);
+  start_step_over ();
 
   /* If we either finished a single-step or hit a breakpoint, but
      the user wanted this thread to be stopped, pretend we got a