[PATCH 2/6] PR breakpoints/7143 - Watchpoint does not trigger when first set

Pedro Alves
2014-02-25T20:32:52Z

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/
2014-02-25 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_over_breakpoint_at): New declaration.
* infrun.c (step_over_aspace, step_over_address): New globals.
(stepping_over_breakpoint_at): New function.
(handle_inferior_event): Clear step_over_aspace and
step_over_address when trap_expected is cleared.
(proceed): Adjust step-over handling to set or clear
step_over_aspace and step_over_address instead of removing all
breakpoints.
(handle_signal_stop): Clear step_over_aspace and step_over_address
when trap_expected is cleared.
(process_event_stop_test): Clear step_over_aspace,
step_over_address and trap_expected if handling a GDB_SIGNAL_TRAP.
Clear step_over_aspace and step_over_address when trap_expected is
cleared.
(proceed): Adjust step-over handling to set step_over_aspace and
step_over_address instead of removing all breakpoints.

gdb/testsuite/
2014-02-25 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/breakpoint.c | 19 ++--
gdb/breakpoint.h | 9 ++
gdb/inferior.h | 6 ++
gdb/infrun.c | 170 +++++++++++++++++++++-------------
gdb/testsuite/gdb.base/watchpoint.exp | 13 +--
gdb/testsuite/gdb.cp/annota2.exp | 3 -
gdb/testsuite/gdb.cp/annota3.exp | 3 -
7 files changed, 133 insertions(+), 90 deletions(-)

diff --git a/gdb/breakpoint.c b/gdb/breakpoint.c
index ef81443..b462bad 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 +2029,13 @@ should_be_inserted (struct bp_location *bl)
if (bl->pspace->breakpoints_not_allowed)
return 0;

+ /* Don't insert a breakpoint we're trying to step over. */
+ if ((bl->loc_type == bp_loc_software_breakpoint
+ || bl->loc_type == bp_loc_hardware_breakpoint)
+ && stepping_over_breakpoint_at (bl->pspace->aspace,
+ bl->address))
+ return 0;
+
return 1;
}

@@ -6786,12 +6788,9 @@ 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 4be9f23..42b1492 100644
--- a/gdb/breakpoint.h
+++ b/gdb/breakpoint.h
@@ -1134,6 +1134,15 @@ 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 43e035b..37542ef 100644
--- a/gdb/inferior.h
+++ b/gdb/inferior.h
@@ -230,6 +230,12 @@ 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 over a breakpoint at ADDRESS
+ in ASPACE. */
+
+extern int stepping_over_breakpoint_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 5d60a90..25b40fb 100644
--- a/gdb/infrun.c
+++ b/gdb/infrun.c
@@ -985,6 +985,38 @@ static int stepping_past_singlestep_breakpoint;
breakpoint in the thread which hit the breakpoint, but then continue
stepping the thread user has selected. */
static ptid_t deferred_step_ptid;
+
+/* If stepping over a breakpoint (not displaced stepping), this is the
+ address (and address space) where that breakpoint is inserted.
+ When not stepping over a breakpoint, STEP_OVER_ASPACE is NULL.
+
+ (Note: presently GDB can only step over a 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). Even with that, the set of breakpoints being stepped
+ over will normally be much small than the set of all breakpoints,
+ so a separate list instead of a flag in the breakpoint locations
+ themselves saves memory. This 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.) */
+static struct address_space *step_over_aspace;
+static CORE_ADDR step_over_address;
+
+/* See inferior.h. */
+
+int
+stepping_over_breakpoint_at (struct address_space *aspace,
+ CORE_ADDR address)
+{
+ return (step_over_aspace != NULL
+ && breakpoint_address_match (aspace, address,
+ step_over_aspace, step_over_address));
+}
+
\f
/* Displaced stepping. */

@@ -1863,8 +1895,11 @@ a command like `return' or `jump' to continue execution."));
remove_single_step_breakpoints ();
singlestep_breakpoints_inserted_p = 0;

- insert_breakpoints ();
+ step_over_aspace = NULL;
+ step_over_address = 0;
tp->control.trap_expected = 0;
+
+ insert_breakpoints ();
}

if (should_resume)
@@ -2232,22 +2267,26 @@ 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 ();
+
+ step_over_aspace = get_regcache_aspace (regcache);
+ step_over_address = regcache_read_pc (regcache);
+ }
+ else
+ {
+ step_over_aspace = NULL;
+ step_over_address = 0;
}

- /* 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)
{
@@ -3943,6 +3982,8 @@ handle_signal_stop (struct execution_control_state *ecs)
}

ecs->event_thread->control.trap_expected = 0;
+ step_over_aspace = NULL;
+ step_over_address = 0;

context_switch (deferred_step_ptid);
deferred_step_ptid = null_ptid;
@@ -4068,35 +4109,17 @@ 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;
}
}

@@ -4384,6 +4407,8 @@ handle_signal_stop (struct execution_control_state *ecs)
ecs->event_thread->step_after_step_resume_breakpoint = 1;
/* Reset trap_expected to ensure breakpoints are re-inserted. */
ecs->event_thread->control.trap_expected = 0;
+ step_over_aspace = NULL;
+ step_over_address = 0;

/* If we were nexting/stepping some other thread, switch to
it, so that we don't continue it, losing control. */
@@ -4416,6 +4441,8 @@ handle_signal_stop (struct execution_control_state *ecs)
insert_hp_step_resume_breakpoint_at_frame (frame);
/* Reset trap_expected to ensure breakpoints are re-inserted. */
ecs->event_thread->control.trap_expected = 0;
+ step_over_aspace = NULL;
+ step_over_address = 0;
keep_going (ecs);
return;
}
@@ -4461,6 +4488,15 @@ process_event_stop_test (struct execution_control_state *ecs)
frame = get_current_frame ();
gdbarch = get_frame_arch (frame);

+ if (ecs->event_thread->control.trap_expected
+ && ecs->event_thread->suspend.stop_signal == GDB_SIGNAL_TRAP)
+ {
+ /* We got our trap. */
+ ecs->event_thread->control.trap_expected = 0;
+ step_over_aspace = NULL;
+ step_over_address = 0;
+ }
+
what = bpstat_what (ecs->event_thread->control.stop_bpstat);

if (what.call_dummy)
@@ -5324,6 +5360,8 @@ switch_back_to_stepped_thread (struct execution_control_state *ecs)
Clear the trap_expected flag before switching back -- this is
what keep_going would do as well, if we called it. */
ecs->event_thread->control.trap_expected = 0;
+ step_over_aspace = NULL;
+ step_over_address = 0;

if (debug_infrun)
fprintf_unfiltered (gdb_stdlog,
@@ -5782,6 +5820,9 @@ 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)
@@ -5795,33 +5836,32 @@ 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);
+ /* Can't step over more than one breakpoint simultaneously
+ without displaced stepping. */
+ gdb_assert (step_over_aspace == NULL);

- 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 ();
- }
+ step_over_aspace = get_regcache_aspace (regcache);
+ step_over_address = regcache_read_pc (regcache);
}
- else
- {
- volatile struct gdb_exception e;

- /* 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/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