[PATCH] PR gdb/11568 - delete thread-specific breakpoints on thread exit

Pedro Alves
2013-09-02T16:52:15Z

On 09/02/2013 05:46 PM, Pedro Alves wrote:

> Here's the diff I got on top of yours. I'll send out the
> final diff in a bit.

Here it is. Let me know what you all think.

---------
Subject: PR gdb/11568 - delete thread-specific breakpoints on thread exit

Bug 11568 - delete thread-specific breakpoints on thread exit

PR gdb/11568 is about thread-specific breakpoints being left behind
when the corresponding thread exits.

Currently:

(gdb) b start thread 2
Breakpoint 3 at 0x400614: file thread-specific-bp.c, line 23.
(gdb) b end
Breakpoint 4 at 0x40061f: file thread-specific-bp.c, line 29.
(gdb) c
Continuing.
[Thread 0x7ffff7fcb700 (LWP 14925) exited]
[Switching to Thread 0x7ffff7fcc740 (LWP 14921)]

Breakpoint 4, end () at thread-specific-bp.c:29
29 }
(gdb) info threads
Id Target Id Frame
* 1 Thread 0x7ffff7fcc740 (LWP 14921) "thread-specific" end () at thread-specific-bp.c:29
(gdb) info breakpoints
Num Type Disp Enb Address What
2 breakpoint keep y 0x0000000000400614 in start at thread-specific-bp.c:23
breakpoint already hit 1 time
3 breakpoint keep y 0x0000000000400614 in start at thread-specific-bp.c:23 thread 2
stop only in thread 2
4 breakpoint keep y 0x000000000040061f in end at thread-specific-bp.c:29
breakpoint already hit 1 time

Note that the thread-specific breakpoint 3 stayed around, even though
thread 2 is gone.

There's no way that breakpoint can trigger again (*), so the PR argues
that the breakpoint should just be removed, like local watchpoints.
I'm ambivalent on this -- it could be reasonable to disable the
breakpoint (kind of like breakpoint in shared library code when the
DSO is unloaded), so the user could still use it as visual template
for creating other breakpoints (copy/paste command lists, etc.), or we
could have a way to change to which thread a breakpoint applies. But,
several people pushed this direction, and I don't plan on arguing...

(*) - actually, there is ... thread numbers are reset on "run", so
the user could do "break foo thread 2", "run", and expect the
breakpoint to hit again on the second thread. But given gdb's thread
numbering can't really be stable, that'd only work sufficiently well
for thread 1, so we'd better call it unsupported.

So with the patch, whenever a thread is deleted from GDB's list, GDB
goes through the thread-specific breakpoints and deletes corresponding
breakpoints. Since this is user-visible, GDB prints out:

Thread-specific breakpoint 3 deleted - thread 2 is gone.

And of course, we end up with:

(gdb) info breakpoints
Num Type Disp Enb Address What
2 breakpoint keep y 0x0000000000400614 in start at thread-specific-bp.c:23
breakpoint already hit 1 time
4 breakpoint keep y 0x000000000040061f in end at thread-specific-bp.c:29
breakpoint already hit 1 time

2013-09-02 Muhammad Waqas <mwaqas@codesourcery.com>
Pedro Alves <palves@redhat.com>

PR gdb/11568
* breakpoint.c (remove_threaded_breakpoints): New function.
(_initialize_breakpoint): Attach remove_threaded_breakpoints
as thread_exit observer.

2013-09-02 Muhammad Waqas <mwaqas@codesourccery.com>
Jan Kratochvil <jan.kartochvil@redhat.com>
Pedro Alves <palves@redhat.com>

PR gdb/11568
* gdb.thread/thread-specific-bp.c: New file.
* gdb.thread/thread-specific-bp.exp: New file.
---

gdb/breakpoint.c | 25 ++++
gdb/testsuite/gdb.threads/thread-specific-bp.c | 39 +++++++
gdb/testsuite/gdb.threads/thread-specific-bp.exp | 127 ++++++++++++++++++++++
3 files changed, 191 insertions(+)
create mode 100644 gdb/testsuite/gdb.threads/thread-specific-bp.c
create mode 100644 gdb/testsuite/gdb.threads/thread-specific-bp.exp

diff --git a/gdb/breakpoint.c b/gdb/breakpoint.c
index 44bb7a8..734dfd6 100644
--- a/gdb/breakpoint.c
+++ b/gdb/breakpoint.c
@@ -2928,6 +2928,30 @@ remove_breakpoints (void)
return val;
}

+/* When a thread exits, remove breakpoints that are related to
+ that thread. */
+
+static void
+remove_threaded_breakpoints (struct thread_info *tp, int silent)
+{
+ struct breakpoint *b, *b_tmp;
+
+ ALL_BREAKPOINTS_SAFE (b, b_tmp)
+ {
+ if (b->thread == tp->num)
+ {
+ b->disposition = disp_del_at_next_stop;
+
+ printf_filtered (_("\
+Thread-specific breakpoint %d deleted - thread %d is gone.\n"),
+ b->number, tp->num);
+
+ /* Hide it from the user. */
+ b->number = 0;
+ }
+ }
+}
+
/* Remove breakpoints of process PID. */

int
@@ -16587,4 +16611,5 @@ agent-printf \"printf format string\", arg1, arg2, arg3, ..., argn\n\
automatic_hardware_breakpoints = 1;

observer_attach_about_to_proceed (breakpoint_about_to_proceed);
+ observer_attach_thread_exit (remove_threaded_breakpoints);
}
diff --git a/gdb/testsuite/gdb.threads/thread-specific-bp.c b/gdb/testsuite/gdb.threads/thread-specific-bp.c
new file mode 100644
index 0000000..5ca02c7
--- /dev/null
+++ b/gdb/testsuite/gdb.threads/thread-specific-bp.c
@@ -0,0 +1,39 @@
+/* This testcase is part of GDB, the GNU debugger.
+
+ Copyright 2013 Free Software Foundation, Inc.
+
+ This program is free software; you can redistribute it and/or modify
+ it under the terms of the GNU General Public License as published by
+ the Free Software Foundation; either version 3 of the License, or
+ (at your option) any later version.
+
+ This program is distributed in the hope that it will be useful,
+ but WITHOUT ANY WARRANTY; without even the implied warranty of
+ MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+ GNU General Public License for more details.
+
+ You should have received a copy of the GNU General Public License
+ along with this program. If not, see <http://www.gnu.org/licenses/>. */
+
+#include <pthread.h>
+
+static void *
+start (void *arg)
+{
+ return NULL;
+}
+
+static void
+end (void)
+{
+}
+
+int
+main (void)
+{
+ pthread_t thread;
+ pthread_create (&thread, NULL, start, NULL);
+ pthread_join (thread, NULL);
+ end ();
+ return 0;
+}
diff --git a/gdb/testsuite/gdb.threads/thread-specific-bp.exp b/gdb/testsuite/gdb.threads/thread-specific-bp.exp
new file mode 100644
index 0000000..9f7d142
--- /dev/null
+++ b/gdb/testsuite/gdb.threads/thread-specific-bp.exp
@@ -0,0 +1,127 @@
+# Copyright (C) 2013 Free Software Foundation, Inc.
+
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program. If not, see <http://www.gnu.org/licenses/>.
+#
+# Verify that a thread-specific breakpoint is deleted when the
+# corresponding thread is gone.
+
+standard_testfile
+
+if {[gdb_compile_pthreads \
+ "${srcdir}/${subdir}/${srcfile}" \
+ "${binfile}" executable {debug} ] != "" } {
+ return -1
+}
+
+clean_restart ${binfile}
+
+# Extract and return the thread ID of the thread stopped at function
+# FUNC.
+
+proc get_thread_id {func} {
+ global gdb_prompt
+
+ set thre -1
+ set test "get $func thread id"
+ gdb_test_multiple "info threads" $test {
+ -re "(\[0-9\]+)\[^\n\r\]*Thread\[^\n\r\]*$func.*$gdb_prompt $" {
+ # Get the thread's id.
+ set thre $expect_out(1,string)
+ pass $test
+ }
+ }
+
+ return $thre
+}
+
+proc check_thread_specific_breakpoint {mode} {
+ with_test_prefix "$mode" {
+ global gdb_prompt
+
+ if ![runto_main] {
+ untested "could not run to main"
+ return -1
+ }
+
+ set main_thre [get_thread_id "main"]
+ if { $main_thre < 0 } {
+ return -1
+ }
+
+ gdb_breakpoint "start"
+ gdb_continue_to_breakpoint "start"
+
+ set start_thre [get_thread_id "start"]
+ if { $start_thre < 0 } {
+ return -1
+ }
+
+ # Set a thread-specific breakpoint at "main". This can't ever
+ # be hit, but that's OK.
+ gdb_breakpoint "main thread $start_thre"
+ gdb_test "info break" ".*breakpoint.*thread $start_thre" "breakpoint set"
+
+ # Set breakpoint at a place only reacheable after the "start"
+ # thread exits.
+ gdb_breakpoint "end"
+
+ # Switch back to the main thread, and resume all threads. The
+ # "start" thread exits, and the main thread reaches "end".
+ gdb_test "thread $main_thre" \
+ "Switching to thread $main_thre.*" \
+ "thread $main_thre selected"
+
+ if { $mode == "non-stop" } {
+ set cmd "continue -a"
+ } else {
+ set cmd "continue"
+ }
+ gdb_test "$cmd" \
+ "Breakpoint .* end .* at .*" \
+ "continue to end"
+
+ # Force GDB to update the thread list. Otherwise, depending
+ # on target, GDB may not realize that the start thread has
+ # exited and thus not remove the thread specific breakpoint.
+ set test "thread start is gone"
+ gdb_test_multiple "info threads" $test {
+ -re "\[0-9\]+.*start.*$gdb_prompt $" {
+ fail $test
+ }
+ -re "$gdb_prompt $" {
+ pass $test
+ }
+ }
+
+ set test "thread-specific breakpoint was deleted"
+ gdb_test_multiple "info breakpoint" $test {
+ -re "thread $start_thre\n$gdb_prompt $" {
+ fail $test
+ }
+ -re "$gdb_prompt $" {
+ pass $test
+ }
+ }
+ }
+}
+
+# Test all-stop mode.
+check_thread_specific_breakpoint "all-stop"
+
+clean_restart ${binfile}
+
+# Test non-stop mode.
+gdb_test_no_output "set target-async on" "set async mode"
+gdb_test_no_output "set non-stop on" "set non-stop mode"
+check_thread_specific_breakpoint "non-stop"