[PATCH v2 2/2] btrace: allow recording to be started for running threads

[Markus Metzger <markus.t.metzger@intel.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.  Skip that step to allow recording to be started while
threads are running.

If we do run into errors, we need to undo the tracing enable to not leak this
thread.  The operation did not complete so our caller won't clean up this
thread.

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.

CC:  Simon Marchi  <simon.marchi@ericsson.com>

2017-01-20  Markus Metzger  <markus.t.metzger@intel.com>

gdb/
	* btrace.c (btrace_enable): Do not call btrace_add_pc for
	BTRACE_FORMAT_PT or if can_access_registers_ptid returns false.
	(validate_registers_access_ptid): New.
	(btrace_add_pc): Call validate_registers_access_ptid.

testsuite/
	* gdb.btrace/enable-running.c: New.
	* gdb.btrace/enable-running.exp: New.
---
 gdb/btrace.c                                | 44 ++++++++++++++++--
 gdb/testsuite/gdb.btrace/enable-running.c   | 47 +++++++++++++++++++
 gdb/testsuite/gdb.btrace/enable-running.exp | 72 +++++++++++++++++++++++++++++
 3 files changed, 159 insertions(+), 4 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 d266af7..4380e6d 100644
--- a/gdb/btrace.c
+++ b/gdb/btrace.c
@@ -1422,6 +1422,18 @@ btrace_compute_ftrace (struct thread_info *tp, struct btrace_data *btrace)
   do_cleanups (old_chain);
 }
 
+/* Validate that we can read PTID's registers.  */
+
+static void
+validate_registers_access_ptid (ptid_t ptid)
+{
+  struct cleanup *cleanup = save_inferior_ptid ();
+
+  inferior_ptid = ptid;
+  validate_registers_access ();
+  do_cleanups (cleanup);
+}
+
 /* Add an entry for the current PC.  */
 
 static void
@@ -1433,6 +1445,9 @@ btrace_add_pc (struct thread_info *tp)
   struct cleanup *cleanup;
   CORE_ADDR pc;
 
+  /* Make sure we can read TP's registers.  */
+  validate_registers_access_ptid (tp->ptid);
+
   regcache = get_thread_regcache (tp->ptid);
   pc = regcache_read_pc (regcache);
 
@@ -1472,10 +1487,31 @@ btrace_enable (struct thread_info *tp, const struct btrace_config *conf)
 
   tp->btrace.target = target_enable_btrace (tp->ptid, 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);
+  /* We're done if we failed to enable tracing.  */
+  if (tp->btrace.target == NULL)
+    return;
+
+  /* We need to undo the enable in case of errors.  */
+  TRY
+    {
+      /* Add an entry for the current PC so we start tracing from where we
+	 enabled it.
+
+	 If we can't access TP's registers, TP is most likely running.  In this
+	 case, we can't really say where tracing was enabled so it should be
+	 safe to simply skip this step.
+
+	 This is not relevant for BTRACE_FORMAT_PT since the trace will already
+	 start at the PC at which tracing was enabled.  */
+      if ((conf->format != BTRACE_FORMAT_PT)
+	  && can_access_registers_ptid (tp->ptid))
+	btrace_add_pc (tp);
+    }
+  CATCH (exception, RETURN_MASK_ALL)
+    {
+      btrace_disable (tp);
+    }
+  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..9fd9aca
--- /dev/null
+++ b/gdb/testsuite/gdb.btrace/enable-running.c
@@ -0,0 +1,47 @@
+/* This testcase is part of GDB, the GNU debugger.
+
+   Copyright 2017 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..577c319
--- /dev/null
+++ b/gdb/testsuite/gdb.btrace/enable-running.exp
@@ -0,0 +1,72 @@
+# This testcase is part of GDB, the GNU debugger.
+#
+# Copyright 2017 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
+
+    with_test_prefix "thread $thread" {
+        gdb_test "thread $thread" "(running).*" "is running"
+        # Stop the thread before reading the trace.
+        gdb_test_multiple "interrupt" "interrupt" {
+            -re "interrupt\r\n$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