[PATCH 2/2] btrace: allow recording to be started for running threads

[Markus Metzger <markus dot t dot metzger at intel dot com>]

When recording is started for a running thread, GDB was able to start tracing
but then failed to read registers to insert the initial entry for the current
PC.  We don't really need that initial entry if we don't know where exactly we
started recording.  Silently ignore such errors to allow recording to be started
while threads are running.

For the BTRACE_FORMAT_PT btrace format, we don't need that initial entry since
it will be recorded in the trace.  We can omit the call to btrace_add_pc.

2016-12-06  Markus Metzger  <markus.t.metzger@intel.com>

gdb/
	* btrace.c (btrace_enable): Do not call btrace_add_pc for
	BTRACE_FORMAT_PT.  Silently ignore errors from btrace_add_pc.

testsuite/
	* gdb.btrace/enable-running.c: New.
	* gdb.btrace/enable-running.exp: New.
---
 gdb/btrace.c                                | 24 +++++++++-
 gdb/testsuite/gdb.btrace/enable-running.c   | 47 +++++++++++++++++++
 gdb/testsuite/gdb.btrace/enable-running.exp | 72 +++++++++++++++++++++++++++++
 3 files changed, 141 insertions(+), 2 deletions(-)
 create mode 100644 gdb/testsuite/gdb.btrace/enable-running.c
 create mode 100644 gdb/testsuite/gdb.btrace/enable-running.exp

diff --git a/gdb/btrace.c b/gdb/btrace.c
index 39d537c..920b9ab 100644
--- a/gdb/btrace.c
+++ b/gdb/btrace.c
@@ -1474,8 +1474,28 @@ btrace_enable (struct thread_info *tp, const struct btrace_config *conf)
 
   /* Add an entry for the current PC so we start tracing from where we
      enabled it.  */
-  if (tp->btrace.target != NULL)
-    btrace_add_pc (tp);
+  TRY
+    {
+      /* This is not relevant for BTRACE_FORMAT_PT since the trace will already
+	 start at the enable location.  */
+      if ((tp->btrace.target != NULL) && (conf->format != BTRACE_FORMAT_PT))
+	btrace_add_pc (tp);
+    }
+  CATCH (exception, RETURN_MASK_ALL)
+    {
+      /* We may fail to add the initial entry, for example if TP is currently
+	 running.
+
+	 We won't be able to stitch the initial trace chunk to this initial
+	 entry so tracing will start at the next branch target instead of at the
+	 current PC.  Since TP is currently running, this shouldn't make a
+	 difference.
+
+	 If TP were waiting most of the time and made only a little bit of
+	 progress before it was stopped, we'd lose the instructions until the
+	 first branch.  */
+    }
+  END_CATCH
 }
 
 /* See btrace.h.  */
diff --git a/gdb/testsuite/gdb.btrace/enable-running.c b/gdb/testsuite/gdb.btrace/enable-running.c
new file mode 100644
index 0000000..6380c29
--- /dev/null
+++ b/gdb/testsuite/gdb.btrace/enable-running.c
@@ -0,0 +1,47 @@
+/* This testcase is part of GDB, the GNU debugger.
+
+   Copyright 2016 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 int global;
+
+static void *
+test (void *arg)
+{
+  for (;;)
+    global += 1;
+
+  return arg;
+}
+
+int
+main (void)
+{
+  int i;
+
+  for (i = 0; i < 3; ++i)
+    {
+      pthread_t th;
+
+      pthread_create (&th, NULL, test, NULL);
+      pthread_detach (th);
+    }
+
+  test (NULL); /* bp.1 */
+
+  return 0;
+}
diff --git a/gdb/testsuite/gdb.btrace/enable-running.exp b/gdb/testsuite/gdb.btrace/enable-running.exp
new file mode 100644
index 0000000..ead86af
--- /dev/null
+++ b/gdb/testsuite/gdb.btrace/enable-running.exp
@@ -0,0 +1,72 @@
+# This testcase is part of GDB, the GNU debugger.
+#
+# Copyright 2016 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/>.
+
+if { [skip_btrace_tests] } { return -1 }
+
+standard_testfile
+if {[gdb_compile_pthreads "$srcdir/$subdir/$srcfile" "$binfile" executable {debug}] != "" } {
+    return -1
+}
+clean_restart $testfile
+
+# We need to enable non-stop mode for the remote case.
+gdb_test_no_output "set non-stop on"
+
+if ![runto_main] {
+    return -1
+}
+
+set bp_1 [gdb_get_line_number "bp.1" $srcfile]
+
+gdb_breakpoint $bp_1
+gdb_continue_to_breakpoint "cont to $bp_1" ".*$bp_1\r\n.*"
+gdb_test "cont&" "Continuing\."
+
+# All threads are running.  Let's start recording.
+gdb_test_no_output "record btrace"
+
+proc check_tracing_enabled { thread } {
+    global gdb_prompt
+
+    gdb_test "thread $thread" "(running).*"
+    with_test_prefix "thread $thread" {
+        # Stop the thread before reading the trace.
+        gdb_test_multiple "interrupt" "interrupt" {
+            -re "interrupt\r\n$gdb_prompt " {
+                pass "interrupt"
+            }
+        }
+        # Wait until the thread actually stopped.
+        gdb_test_multiple "" "stopped" {
+            -re "Thread $thread.*stopped\." {
+                pass "stopped"
+            }
+        }
+        # We will consume the thread's current location as part of the
+        # "info record" output.
+        gdb_test "info record" [multi_line \
+            "Active record target: record-btrace" \
+            "Recording format: .*" \
+            "Recorded \[0-9\]+ instructions \[^\\\r\\\n\]*" \
+        ]
+    }
+}
+
+# Check that recording was started on each thread.
+foreach thread {1 2 3 4} {
+    check_tracing_enabled $thread
+}
-- 
1.8.3.1