Re: [PATCH v3 2/5] PR breakpoints/7143 - Watchpoint does not trigger when first set

[Pedro Alves <palves at redhat dot com>]

On 03/17/2014 05:11 PM, Pedro Alves wrote:
> On 03/16/2014 03:41 AM, Doug Evans wrote:
> 
>> Hi.  Ok by me with one nit.
> 
> Thanks.  FAOD, are you planning on replying to the other patches
> in the series?
> 
>>> +   Presently GDB can only step over a breakpoint at any given time.
>>
>> I realize this is nitpicky, but can you reword this as:
> 
> Sure thing.

Here's what I pushed (nothing else changed).

Many thanks for the review.

-----
PR breakpoints/7143 - Watchpoint does not trigger when first
 set

Say the program is stopped at a breakpoint, and the user sets a
watchpoint.  When the program is next resumed, GDB will first step
over the breakpoint, as explained in the manual:

  @value {GDBN} normally ignores breakpoints when it resumes
  execution, until at least one instruction has been executed.  If it
  it did not do this, you would be unable to proceed past a breakpoint
  without first disabling the breakpoint.  This rule applies whether
  or not the breakpoint already existed when your program stopped.

However, GDB currently also removes watchpoints, catchpoints, etc.,
and that means that the first instruction off the breakpoint does not
trigger the watchpoint, catchpoint, etc.

testsuite/gdb.base/watchpoint.exp has a kfail for this.

The PR proposes installing watchpoints only when stepping over a
breakpoint, but that misses catchpoints, etc.

A better fix would instead work from the opposite direction -- remove
only real breakpoints, leaving all other kinds of breakpoints
inserted.

But, going further, it's really a waste to constantly remove/insert
all breakpoints when stepping over a single breakpoint (generating a
pair of RSP z/Z packets for each breakpoint), so the fix goes a step
further and makes GDB remove _only_ the breakpoint being stepped over,
leaving all others installed.  This then has the added benefit of
reducing breakpoint-related RSP traffic substancialy when there are
many breakpoints set.

gdb/
2014-03-20  Pedro Alves  <palves@redhat.com>

	PR breakpoints/7143
	* breakpoint.c (should_be_inserted): Don't insert breakpoints that
	are being stepped over.
	(breakpoint_address_match): Make extern.
	* breakpoint.h (breakpoint_address_match): New declaration.
	* inferior.h (stepping_past_instruction_at): New declaration.
	* infrun.c (struct step_over_info): New type.
	(step_over_info): New global.
	(set_step_over_info, clear_step_over_info)
	(stepping_past_instruction_at): New functions.
	(handle_inferior_event): Clear the step-over info when
	trap_expected is cleared.
	(resume): Remove now stale comment.
	(clear_proceed_status): Clear step-over info.
	(proceed): Adjust step-over handling to set or clear the step-over
	info instead of removing all breakpoints.
	(handle_signal_stop): When setting up a thread-hop, don't remove
	breakpoints here.
	(stop_stepping): Clear step-over info.
	(keep_going): Adjust step-over handling to set or clear step-over
	info and then always inserting breakpoints, instead of removing
	all breakpoints when stepping over one.

gdb/testsuite/
2014-03-20  Pedro Alves  <palves@redhat.com>

	PR breakpoints/7143
	* gdb.base/watchpoint.exp: Mention bugzilla bug number instead of
	old gnats gdb/38.  Remove kfail.  Adjust to use gdb_test instead
	of gdb_test_multiple.
	* gdb.cp/annota2.exp: Remove kfail for gdb/38.
	* gdb.cp/annota3.exp: Remove kfail for gdb/38.
---
 gdb/ChangeLog                         |  25 +++++
 gdb/breakpoint.c                      |  20 ++--
 gdb/breakpoint.h                      |  10 ++
 gdb/inferior.h                        |   6 +
 gdb/infrun.c                          | 199 +++++++++++++++++++++-------------
 gdb/testsuite/ChangeLog               |   9 ++
 gdb/testsuite/gdb.base/watchpoint.exp |  13 +--
 gdb/testsuite/gdb.cp/annota2.exp      |   3 -
 gdb/testsuite/gdb.cp/annota3.exp      |   3 -
 9 files changed, 189 insertions(+), 99 deletions(-)

diff --git a/gdb/ChangeLog b/gdb/ChangeLog
index 18244d2..7f99a5a 100644
--- a/gdb/ChangeLog
+++ b/gdb/ChangeLog
@@ -1,5 +1,30 @@
 2014-03-20  Pedro Alves  <palves@redhat.com>
 
+	PR breakpoints/7143
+	* breakpoint.c (should_be_inserted): Don't insert breakpoints that
+	are being stepped over.
+	(breakpoint_address_match): Make extern.
+	* breakpoint.h (breakpoint_address_match): New declaration.
+	* inferior.h (stepping_past_instruction_at): New declaration.
+	* infrun.c (struct step_over_info): New type.
+	(step_over_info): New global.
+	(set_step_over_info, clear_step_over_info)
+	(stepping_past_instruction_at): New functions.
+	(handle_inferior_event): Clear the step-over info when
+	trap_expected is cleared.
+	(resume): Remove now stale comment.
+	(clear_proceed_status): Clear step-over info.
+	(proceed): Adjust step-over handling to set or clear the step-over
+	info instead of removing all breakpoints.
+	(handle_signal_stop): When setting up a thread-hop, don't remove
+	breakpoints here.
+	(stop_stepping): Clear step-over info.
+	(keep_going): Adjust step-over handling to set or clear step-over
+	info and then always inserting breakpoints, instead of removing
+	all breakpoints when stepping over one.
+
+2014-03-20  Pedro Alves  <palves@redhat.com>
+
 	* infrun.c (previous_inferior_ptid): Adjust comment.
 	(deferred_step_ptid): Delete.
 	(infrun_thread_ptid_changed, prepare_to_proceed)
diff --git a/gdb/breakpoint.c b/gdb/breakpoint.c
index 1551b99..03b21cb 100644
--- a/gdb/breakpoint.c
+++ b/gdb/breakpoint.c
@@ -165,11 +165,6 @@ static void describe_other_breakpoints (struct gdbarch *,
 					struct program_space *, CORE_ADDR,
 					struct obj_section *, int);
 
-static int breakpoint_address_match (struct address_space *aspace1,
-				     CORE_ADDR addr1,
-				     struct address_space *aspace2,
-				     CORE_ADDR addr2);
-
 static int watchpoint_locations_match (struct bp_location *loc1,
 				       struct bp_location *loc2);
 
@@ -2034,6 +2029,14 @@ should_be_inserted (struct bp_location *bl)
   if (bl->pspace->breakpoints_not_allowed)
     return 0;
 
+  /* Don't insert a breakpoint if we're trying to step past its
+     location.  */
+  if ((bl->loc_type == bp_loc_software_breakpoint
+       || bl->loc_type == bp_loc_hardware_breakpoint)
+      && stepping_past_instruction_at (bl->pspace->aspace,
+				       bl->address))
+    return 0;
+
   return 1;
 }
 
@@ -6792,12 +6795,9 @@ watchpoint_locations_match (struct bp_location *loc1,
 	  && loc1->length == loc2->length);
 }
 
-/* Returns true if {ASPACE1,ADDR1} and {ASPACE2,ADDR2} represent the
-   same breakpoint location.  In most targets, this can only be true
-   if ASPACE1 matches ASPACE2.  On targets that have global
-   breakpoints, the address space doesn't really matter.  */
+/* See breakpoint.h.  */
 
-static int
+int
 breakpoint_address_match (struct address_space *aspace1, CORE_ADDR addr1,
 			  struct address_space *aspace2, CORE_ADDR addr2)
 {
diff --git a/gdb/breakpoint.h b/gdb/breakpoint.h
index bf1f52a..b4abcb8 100644
--- a/gdb/breakpoint.h
+++ b/gdb/breakpoint.h
@@ -1135,6 +1135,16 @@ extern int hardware_watchpoint_inserted_in_range (struct address_space *,
 extern int breakpoint_thread_match (struct address_space *, 
 				    CORE_ADDR, ptid_t);
 
+/* Returns true if {ASPACE1,ADDR1} and {ASPACE2,ADDR2} represent the
+   same breakpoint location.  In most targets, this can only be true
+   if ASPACE1 matches ASPACE2.  On targets that have global
+   breakpoints, the address space doesn't really matter.  */
+
+extern int breakpoint_address_match (struct address_space *aspace1,
+				     CORE_ADDR addr1,
+				     struct address_space *aspace2,
+				     CORE_ADDR addr2);
+
 extern void until_break_command (char *, int, int);
 
 /* Initialize a struct bp_location.  */
diff --git a/gdb/inferior.h b/gdb/inferior.h
index b15f692..64a78ce 100644
--- a/gdb/inferior.h
+++ b/gdb/inferior.h
@@ -222,6 +222,12 @@ void set_step_info (struct frame_info *frame, struct symtab_and_line sal);
 
 extern void clear_exit_convenience_vars (void);
 
+/* Returns true if we're trying to step past the instruction at
+   ADDRESS in ASPACE.  */
+
+extern int stepping_past_instruction_at (struct address_space *aspace,
+					 CORE_ADDR address);
+
 /* From infcmd.c */
 
 extern void post_create_inferior (struct target_ops *, int);
diff --git a/gdb/infrun.c b/gdb/infrun.c
index f189a86..ed8ec30 100644
--- a/gdb/infrun.c
+++ b/gdb/infrun.c
@@ -977,6 +977,76 @@ static CORE_ADDR singlestep_pc;
 static ptid_t saved_singlestep_ptid;
 static int stepping_past_singlestep_breakpoint;
 
+/* Info about an instruction that is being stepped over.  Invalid if
+   ASPACE is NULL.  */
+
+struct step_over_info
+{
+  /* The instruction's address space.  */
+  struct address_space *aspace;
+
+  /* The instruction's address.  */
+  CORE_ADDR address;
+};
+
+/* The step-over info of the location that is being stepped over.
+
+   Note that with async/breakpoint always-inserted mode, a user might
+   set a new breakpoint/watchpoint/etc. exactly while a breakpoint is
+   being stepped over.  As setting a new breakpoint inserts all
+   breakpoints, we need to make sure the breakpoint being stepped over
+   isn't inserted then.  We do that by only clearing the step-over
+   info when the step-over is actually finished (or aborted).
+
+   Presently GDB can only step over one breakpoint at any given time.
+   Given threads that can't run code in the same address space as the
+   breakpoint's can't really miss the breakpoint, GDB could be taught
+   to step-over at most one breakpoint per address space (so this info
+   could move to the address space object if/when GDB is extended).
+   The set of breakpoints being stepped over will normally be much
+   smaller than the set of all breakpoints, so a flag in the
+   breakpoint location structure would be wasteful.  A separate list
+   also saves complexity and run-time, as otherwise we'd have to go
+   through all breakpoint locations clearing their flag whenever we
+   start a new sequence.  Similar considerations weigh against storing
+   this info in the thread object.  Plus, not all step overs actually
+   have breakpoint locations -- e.g., stepping past a single-step
+   breakpoint, or stepping to complete a non-continuable
+   watchpoint.  */
+static struct step_over_info step_over_info;
+
+/* Record the address of the breakpoint/instruction we're currently
+   stepping over.  */
+
+static void
+set_step_over_info (struct address_space *aspace, CORE_ADDR address)
+{
+  step_over_info.aspace = aspace;
+  step_over_info.address = address;
+}
+
+/* Called when we're not longer stepping over a breakpoint / an
+   instruction, so all breakpoints are free to be (re)inserted.  */
+
+static void
+clear_step_over_info (void)
+{
+  step_over_info.aspace = NULL;
+  step_over_info.address = 0;
+}
+
+/* See inferior.h.  */
+
+int
+stepping_past_instruction_at (struct address_space *aspace,
+			      CORE_ADDR address)
+{
+  return (step_over_info.aspace != NULL
+	  && breakpoint_address_match (aspace, address,
+				       step_over_info.aspace,
+				       step_over_info.address));
+}
+
 
 /* Displaced stepping.  */
 
@@ -1852,8 +1922,10 @@ a command like `return' or `jump' to continue execution."));
       remove_single_step_breakpoints ();
       singlestep_breakpoints_inserted_p = 0;
 
-      insert_breakpoints ();
+      clear_step_over_info ();
       tp->control.trap_expected = 0;
+
+      insert_breakpoints ();
     }
 
   if (should_resume)
@@ -1894,12 +1966,7 @@ a command like `return' or `jump' to continue execution."));
 	     hit, by single-stepping the thread with the breakpoint
 	     removed.  In which case, we need to single-step only this
 	     thread, and keep others stopped, as they can miss this
-	     breakpoint if allowed to run.
-
-	     The current code actually removes all breakpoints when
-	     doing this, not just the one being stepped over, so if we
-	     let other threads run, we can actually miss any
-	     breakpoint, not just the one at PC.  */
+	     breakpoint if allowed to run.  */
 	  resume_ptid = inferior_ptid;
 	}
 
@@ -2031,6 +2098,8 @@ clear_proceed_status (void)
 
   stop_after_trap = 0;
 
+  clear_step_over_info ();
+
   observer_notify_about_to_proceed ();
 
   if (stop_registers)
@@ -2217,22 +2286,23 @@ proceed (CORE_ADDR addr, enum gdb_signal siggnal, int step)
   tp = inferior_thread ();
 
   if (force_step)
+    tp->control.trap_expected = 1;
+
+  /* If we need to step over a breakpoint, and we're not using
+     displaced stepping to do so, insert all breakpoints (watchpoints,
+     etc.) but the one we're stepping over, step one instruction, and
+     then re-insert the breakpoint when that step is finished.  */
+  if (tp->control.trap_expected && !use_displaced_stepping (gdbarch))
     {
-      tp->control.trap_expected = 1;
-      /* If displaced stepping is enabled, we can step over the
-	 breakpoint without hitting it, so leave all breakpoints
-	 inserted.  Otherwise we need to disable all breakpoints, step
-	 one instruction, and then re-add them when that step is
-	 finished.  */
-      if (!use_displaced_stepping (gdbarch))
-	remove_breakpoints ();
+      struct regcache *regcache = get_current_regcache ();
+
+      set_step_over_info (get_regcache_aspace (regcache),
+			  regcache_read_pc (regcache));
     }
+  else
+    clear_step_over_info ();
 
-  /* We can insert breakpoints if we're not trying to step over one,
-     or if we are stepping over one but we're using displaced stepping
-     to do so.  */
-  if (! tp->control.trap_expected || use_displaced_stepping (gdbarch))
-    insert_breakpoints ();
+  insert_breakpoints ();
 
   if (!non_stop)
     {
@@ -3992,9 +4062,6 @@ handle_signal_stop (struct execution_control_state *ecs)
 
       if (thread_hop_needed)
 	{
-	  struct regcache *thread_regcache;
-	  int remove_status = 0;
-
 	  if (debug_infrun)
 	    fprintf_unfiltered (gdb_stdlog, "infrun: thread_hop_needed\n");
 
@@ -4013,35 +4080,17 @@ handle_signal_stop (struct execution_control_state *ecs)
 	      singlestep_breakpoints_inserted_p = 0;
 	    }
 
-	  /* If the arch can displace step, don't remove the
-	     breakpoints.  */
-	  thread_regcache = get_thread_regcache (ecs->ptid);
-	  if (!use_displaced_stepping (get_regcache_arch (thread_regcache)))
-	    remove_status = remove_breakpoints ();
-
-	  /* Did we fail to remove breakpoints?  If so, try
-	     to set the PC past the bp.  (There's at least
-	     one situation in which we can fail to remove
-	     the bp's: On HP-UX's that use ttrace, we can't
-	     change the address space of a vforking child
-	     process until the child exits (well, okay, not
-	     then either :-) or execs.  */
-	  if (remove_status != 0)
-	    error (_("Cannot step over breakpoint hit in wrong thread"));
-	  else
-	    {			/* Single step */
-	      if (!non_stop)
-		{
-		  /* Only need to require the next event from this
-		     thread in all-stop mode.  */
-		  waiton_ptid = ecs->ptid;
-		  infwait_state = infwait_thread_hop_state;
-		}
-
-	      ecs->event_thread->stepping_over_breakpoint = 1;
-	      keep_going (ecs);
-	      return;
+	  if (!non_stop)
+	    {
+	      /* Only need to require the next event from this thread
+		 in all-stop mode.  */
+	      waiton_ptid = ecs->ptid;
+	      infwait_state = infwait_thread_hop_state;
 	    }
+
+	  ecs->event_thread->stepping_over_breakpoint = 1;
+	  keep_going (ecs);
+	  return;
 	}
     }
 
@@ -5695,6 +5744,8 @@ stop_stepping (struct execution_control_state *ecs)
   if (debug_infrun)
     fprintf_unfiltered (gdb_stdlog, "infrun: stop_stepping\n");
 
+  clear_step_over_info ();
+
   /* Let callers know we don't want to wait for the inferior anymore.  */
   ecs->wait_some_more = 0;
 }
@@ -5727,6 +5778,9 @@ keep_going (struct execution_control_state *ecs)
     }
   else
     {
+      volatile struct gdb_exception e;
+      struct regcache *regcache = get_current_regcache ();
+
       /* Either the trap was not expected, but we are continuing
 	 anyway (if we got a signal, the user asked it be passed to
 	 the child)
@@ -5740,33 +5794,30 @@ keep_going (struct execution_control_state *ecs)
 	 already inserted breakpoints.  Therefore, we don't
 	 care if breakpoints were already inserted, or not.  */
 
-      if (ecs->event_thread->stepping_over_breakpoint)
+      /* If we need to step over a breakpoint, and we're not using
+	 displaced stepping to do so, insert all breakpoints
+	 (watchpoints, etc.) but the one we're stepping over, step one
+	 instruction, and then re-insert the breakpoint when that step
+	 is finished.  */
+      if (ecs->event_thread->stepping_over_breakpoint
+	  && !use_displaced_stepping (get_regcache_arch (regcache)))
 	{
-	  struct regcache *thread_regcache = get_thread_regcache (ecs->ptid);
-
-	  if (!use_displaced_stepping (get_regcache_arch (thread_regcache)))
-	    {
-	      /* Since we can't do a displaced step, we have to remove
-		 the breakpoint while we step it.  To keep things
-		 simple, we remove them all.  */
-	      remove_breakpoints ();
-	    }
+	  set_step_over_info (get_regcache_aspace (regcache),
+			      regcache_read_pc (regcache));
 	}
       else
-	{
-	  volatile struct gdb_exception e;
+	clear_step_over_info ();
 
-	  /* Stop stepping if inserting breakpoints fails.  */
-	  TRY_CATCH (e, RETURN_MASK_ERROR)
-	    {
-	      insert_breakpoints ();
-	    }
-	  if (e.reason < 0)
-	    {
-	      exception_print (gdb_stderr, e);
-	      stop_stepping (ecs);
-	      return;
-	    }
+      /* Stop stepping if inserting breakpoints fails.  */
+      TRY_CATCH (e, RETURN_MASK_ERROR)
+	{
+	  insert_breakpoints ();
+	}
+      if (e.reason < 0)
+	{
+	  exception_print (gdb_stderr, e);
+	  stop_stepping (ecs);
+	  return;
 	}
 
       ecs->event_thread->control.trap_expected
diff --git a/gdb/testsuite/ChangeLog b/gdb/testsuite/ChangeLog
index 1986747..ee71b74 100644
--- a/gdb/testsuite/ChangeLog
+++ b/gdb/testsuite/ChangeLog
@@ -1,5 +1,14 @@
 2014-03-20  Pedro Alves  <palves@redhat.com>
 
+	PR breakpoints/7143
+	* gdb.base/watchpoint.exp: Mention bugzilla bug number instead of
+	old gnats gdb/38.  Remove kfail.  Adjust to use gdb_test instead
+	of gdb_test_multiple.
+	* gdb.cp/annota2.exp: Remove kfail for gdb/38.
+	* gdb.cp/annota3.exp: Remove kfail for gdb/38.
+
+2014-03-20  Pedro Alves  <palves@redhat.com>
+
 	* gdb.threads/step-over-lands-on-breakpoint.c: New file.
 	* gdb.threads/step-over-lands-on-breakpoint.exp: New file.
 
diff --git a/gdb/testsuite/gdb.base/watchpoint.exp b/gdb/testsuite/gdb.base/watchpoint.exp
index 80d75cb..1123824 100644
--- a/gdb/testsuite/gdb.base/watchpoint.exp
+++ b/gdb/testsuite/gdb.base/watchpoint.exp
@@ -550,21 +550,16 @@ proc test_complex_watchpoint {} {
 proc test_watchpoint_and_breakpoint {} {
     global gdb_prompt
 
-    # This is a test for PR gdb/38, which involves setting a
+    # This is a test for PR breakpoints/7143, which involves setting a
     # watchpoint right after you've reached a breakpoint.
 
     if [runto func3] then {
 	gdb_breakpoint [gdb_get_line_number "second x assignment"]
 	gdb_continue_to_breakpoint "second x assignment"
 	gdb_test "watch x" ".*atchpoint \[0-9\]+: x"
-	gdb_test_multiple "next" "next after watch x" {
-	    -re ".*atchpoint \[0-9\]+: x\r\n\r\nOld value = 0\r\nNew value = 1\r\n.*$gdb_prompt $" {
-		pass "next after watch x"
-	    }
-	    -re "\[0-9\]+\[\t \]+y = 1;\r\n$gdb_prompt $" {
-		kfail "gdb/38" "next after watch x"
-	    }
-	}
+	gdb_test "next" \
+	    ".*atchpoint \[0-9\]+: x\r\n\r\nOld value = 0\r\nNew value = 1\r\n.*" \
+	    "next after watch x"
 
 	gdb_test_no_output "delete \$bpnum" "delete watch x"
     }
diff --git a/gdb/testsuite/gdb.cp/annota2.exp b/gdb/testsuite/gdb.cp/annota2.exp
index 00a6067..6fbf4b5 100644
--- a/gdb/testsuite/gdb.cp/annota2.exp
+++ b/gdb/testsuite/gdb.cp/annota2.exp
@@ -165,9 +165,6 @@ gdb_test_multiple "next" "watch triggered on a.x" {
     -re "\r\n\032\032post-prompt\r\n\r\n\032\032starting\r\n${frames_invalid}${breakpoints_invalid}\r\n\032\032watchpoint 3\r\n.*atchpoint 3: a.x\r\n\r\nOld value = 0\r\nNew value = 1\r\n\r\n\032\032frame-begin 0 $hex\r\n\r\n\032\032frame-function-name\r\nmain\r\n\032\032frame-args\r\n \\(\\)\r\n\032\032frame-source-begin\r\n at \r\n\032\032frame-source-file\r\n.*$srcfile\r\n\032\032frame-source-file-end\r\n:\r\n\032\032frame-source-line\r\n$decimal\r\n\032\032frame-source-end\r\n\r\n\r\n\032\032source .*$srcfile.*beg:$hex\r\n\r\n\032\032frame-end\r\n\r\n\032\032stopped\r\n.*$gdb_prompt$" {
 	pass "watch triggered on a.x"
     }
-    -re "\r\n\032\032post-prompt\r\n\r\n\032\032starting\r\n${frames_invalid}\r\n\032\032source .*$srcfile.*beg:$hex\r\n\r\n\032\032frame-end\r\n\r\n\032\032stopped\r\n$gdb_prompt$" {
-	kfail "gdb/38" "watch triggered on a.x"
-    }
 }
 
 
diff --git a/gdb/testsuite/gdb.cp/annota3.exp b/gdb/testsuite/gdb.cp/annota3.exp
index 957d371..bbf9a1e 100644
--- a/gdb/testsuite/gdb.cp/annota3.exp
+++ b/gdb/testsuite/gdb.cp/annota3.exp
@@ -169,9 +169,6 @@ gdb_test_multiple "next" "watch triggered on a.x" {
     -re "\r\n\032\032post-prompt\r\n\r\n\032\032starting\r\n\r\n\032\032watchpoint 3\r\n.*atchpoint 3: a.x\r\n\r\nOld value = 0\r\nNew value = 1\r\n\r\n(\032\032frame-begin 0 0x\[0-9a-z\]+\r\n|)main \\(\\) at .*$srcfile:$decimal\r\n\r\n\032\032source .*$srcfile.*beg:$hex\r\n\r\n\032\032stopped\r\n.*$gdb_prompt$" { 
 	pass "watch triggered on a.x"
     }
-    -re "\r\n\032\032post-prompt\r\n\r\n\032\032starting\r\n\r\n\032\032source .*$srcfile.*beg:$hex\r\n\r\n\032\032stopped\r\n$gdb_prompt$" {
-	kfail "gdb/38" "watch triggered on a.x"
-    }
 }
 
 #
-- 
1.7.11.7