[PATCH 3.5/6] Multiple Ada task-specific breakpoints at the same address.

Pedro Alves
2014-02-26T13:36:43Z

There's one more...

I already had this patch, but somehow missed that it would be
needed in this series, so left it out. But it turns out that
patch 4 exposes a latent Ada task-specific breakpoints bug,
resulting in a testsuite regression that I failed to notice
when posting this series.

So here's that patch, now with a test that fails against
current mainline, even without the rest of the series.

Is this OK?
------------
Subject: [PATCH] Multiple Ada task-specific breakpoints at the same address.

With the test changed as in the patch, against current mainline, we get:

(gdb) PASS: gdb.ada/tasks.exp: info tasks before inserting breakpoint
break break_me task 1
Breakpoint 2 at 0x4030b0: file /home/pedro/gdb/mygit/src/gdb/testsuite/gdb.ada/tasks/foo.adb, line 27.
(gdb) PASS: gdb.ada/tasks.exp: break break_me task 1
break break_me task 3
Note: breakpoint 2 also set at pc 0x4030b0.
Breakpoint 3 at 0x4030b0: file /home/pedro/gdb/mygit/src/gdb/testsuite/gdb.ada/tasks/foo.adb, line 27.
(gdb) PASS: gdb.ada/tasks.exp: break break_me task 3
continue
Continuing.
[Switching to Thread 0x7ffff7dc7700 (LWP 27133)]

Breakpoint 2, foo.break_me () at /home/pedro/gdb/mygit/src/gdb/testsuite/gdb.ada/tasks/foo.adb:27
27 null;
(gdb) FAIL: gdb.ada/tasks.exp: continue to breakpoint
info tasks
ID TID P-ID Pri State Name
1 63b010 48 Waiting on RV with 3 main_task
2 63bd80 1 48 Accept or Select Term task_list(1)
* 3 63f510 1 48 Accepting RV with 1 task_list(2)
4 642ca0 1 48 Accept or Select Term task_list(3)
(gdb) PASS: gdb.ada/tasks.exp: info tasks after hitting breakpoint

The breakpoint that caused a stop is breakpoint 3, but GDB end up
reporting (and running breakpoint commands of) "Breakpoint 2" instead.

The issue is that the bpstat_check_breakpoint_conditions logic of
"wrong thread" is missing the "wrong task" check. This is usually
harmless, because the thread hop code in infrun.c code that handles
wrong-task-hitting-breakpoint does check for task-specific breakpoints
(within breakpoint_thread_match):

/* Check if a regular breakpoint has been hit before checking
for a potential single step breakpoint. Otherwise, GDB will
not see this breakpoint hit when stepping onto breakpoints. */
if (regular_breakpoint_inserted_here_p (aspace, stop_pc))
{
if (!breakpoint_thread_match (aspace, stop_pc, ecs->ptid))
thread_hop_needed = 1;
}

IOW, usually, when one only has a task specific breakpoint at a given
address, things work correctly. Put another task-specific or
non-task-specific breakpoint there, and things break.

A patch that eliminates the special thread hop code in infrun.c is
what exposed this, as after that GDB solely relies on
bpstat_check_breakpoint_conditions to know whether the right or wrong
task hit a breakpoint. IOW, given the latent bug, Ada task-specific
breakpoints become non-task-specific, and that is caught by the
testsuite, as:

break break_me task 3
Breakpoint 2 at 0x4030b0: file /home/pedro/gdb/mygit/src/gdb/testsuite/gdb.ada/tasks/foo.adb, line 27.
(gdb) PASS: gdb.ada/tasks.exp: break break_me task 3
continue
Continuing.
[Switching to Thread 0x7ffff7fcb700 (LWP 17122)]

Breakpoint 2, foo.break_me () at /home/pedro/gdb/mygit/src/gdb/testsuite/gdb.ada/tasks/foo.adb:27
27 null;
(gdb) PASS: gdb.ada/tasks.exp: continue to breakpoint
info tasks
ID TID P-ID Pri State Name
1 63b010 48 Waiting on RV with 2 main_task
* 2 63bd80 1 48 Accepting RV with 1 task_list(1)
3 63f510 1 48 Accept or Select Term task_list(2)
4 642ca0 1 48 Accept or Select Term task_list(3)
(gdb) FAIL: gdb.ada/tasks.exp: info tasks after hitting breakpoint

It was after seeing this that I thought of how to expose the bug with
current mainline.

Tested on x86_64 Fedora 17.

gdb/
2014-02-26 Pedro Alves <palves@redhat.com>

* breakpoint.c (bpstat_check_breakpoint_conditions): Handle
task-specific breakpoints.

gdb/testsuite/
2014-02-26 Pedro Alves <palves@redhat.com>

* gdb.ada/tasks.exp: Set a task-specific breakpoint at break_me
that won't ever trigger. Make sure that GDB reports the correct
breakpoint that caused the stop.
---
gdb/breakpoint.c | 10 ++++++----
gdb/testsuite/gdb.ada/tasks.exp | 28 ++++++++++++++++++++++++----
2 files changed, 30 insertions(+), 8 deletions(-)

diff --git a/gdb/breakpoint.c b/gdb/breakpoint.c
index b462bad..fd33a3d 100644
--- a/gdb/breakpoint.c
+++ b/gdb/breakpoint.c
@@ -5161,7 +5161,6 @@ bpstat_check_watchpoint (bpstat bs)
static void
bpstat_check_breakpoint_conditions (bpstat bs, ptid_t ptid)
{
- int thread_id = pid_to_thread_id (ptid);
const struct bp_location *bl;
struct breakpoint *b;
int value_is_zero = 0;
@@ -5186,9 +5185,12 @@ bpstat_check_breakpoint_conditions (bpstat bs, ptid_t ptid)
return;
}

- /* If this is a thread-specific breakpoint, don't waste cpu evaluating the
- condition if this isn't the specified thread. */
- if (b->thread != -1 && b->thread != thread_id)
+ /* If this is a thread/task-specific breakpoint, don't waste cpu
+ evaluating the condition if this isn't the specified
+ thread/task. */
+ if ((b->thread != -1 && b->thread != pid_to_thread_id (ptid))
+ || (b->task != 0 && b->task != ada_get_task_number (ptid)))
+
{
bs->stop = 0;
return;
diff --git a/gdb/testsuite/gdb.ada/tasks.exp b/gdb/testsuite/gdb.ada/tasks.exp
index 710deb0..9ffd000 100644
--- a/gdb/testsuite/gdb.ada/tasks.exp
+++ b/gdb/testsuite/gdb.ada/tasks.exp
@@ -37,15 +37,35 @@ gdb_test "info tasks" \
"\r\n"] \
"info tasks before inserting breakpoint"

-# Now, insert a breakpoint that should stop only if task 3 stops.
-gdb_test "break break_me task 3" "Breakpoint .* at .*"
+# Insert a breakpoint that should stop only if task 1 stops. Since
+# task 1 never calls break_me, this shouldn't actually ever trigger.
+# The fact this this breakpoint is created _before_ the next one
+# matter. GDB used to have a bug where it would report the first
+# breakpoint in the list that matched the triggered-breakpoint's
+# address, no matter which task it was specific to.
+gdb_test "break break_me task 1" "Breakpoint .* at .*"
+
+# Now, insert a breakpoint that should stop only if task 3 stops, and
+# extract its number.
+set bp_number -1
+set test "break break_me task 3"
+gdb_test_multiple $test $test {
+ -re "Breakpoint (.*) at .*$gdb_prompt $" {
+ set bp_number $expect_out(1,string)
+ pass $test
+ }
+}
+
+if {$bp_number < 0} {
+ return
+}

# Continue to that breakpoint. Task 2 should hit it first, and GDB
# is expected to ignore that hit and resume the execution. Only then
# task 3 will hit our breakpoint, and GDB is expected to stop at that
-# point.
+# point. Also make sure that GDB reports the correct breakpoint number.
gdb_test "continue" \
- ".*Breakpoint.*, foo.break_me \\(\\).*" \
+ ".*Breakpoint $bp_number, foo.break_me \\(\\).*" \
"continue to breakpoint"

# Check that it is indeed task 3 that hit the breakpoint by checking
--
1.7.11.7