Add a test case for fast tracepoints' locking mechanism

When installing a fast tracepoint, we create a jump pad with a
spin-lock.  This way, only one thread can collect a given tracepoint at
any time.  This test case checks that this lock actually works as
expected.

This test works by creating a function which overrides the in-process
agent library's gdb_collect function.  On start up, GDBserver will ask
GDB with the 'qSymbol' packet about symbols present in the inferior.
GDB will reply with the gdb_agent_gdb_collect function from the test
case instead of the one from the agent.

gdb/testsuite/ChangeLog:

	* gdb.trace/ftrace-lock.c: New file.
	* gdb.trace/ftrace-lock.exp: New file.

1 files changed, 106 insertions, 0 deletions

diff --git a/gdb/testsuite/gdb.trace/ftrace-lock.c b/gdb/testsuite/gdb.trace/ftrace-lock.c
new file mode 100644
index 0000000..8ed45f4
--- /dev/null
+++ b/gdb/testsuite/gdb.trace/ftrace-lock.c
@@ -0,0 +1,106 @@
+/* This testcase is part of GDB, the GNU debugger.
+
+   Copyright 2015 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>
+
+#ifdef SYMBOL_PREFIX
+#define SYMBOL(str)     SYMBOL_PREFIX #str
+#else
+#define SYMBOL(str)     #str
+#endif
+
+/* Called if the testcase failed.  */
+static void
+fail (void)
+{
+}
+
+static pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER;
+
+/* This function overrides gdb_collect in the in-process agent library.
+   See gdbserver/tracepoint.c (gdb_collect).  We want this function to
+   be ran instead of the one from the library to easily check that only
+   one thread is tracing at a time.
+
+   This works as expected because GDBserver will ask GDB about symbols
+   present in the inferior with the 'qSymbol' packet.  And GDB will
+   reply with the address of this function instead of the one from the
+   in-process agent library.  */
+
+void
+gdb_agent_gdb_collect (void *tpoint, unsigned char *regs)
+{
+  /* If we cannot acquire a lock, then this means another thread is
+     tracing and the lock implemented by the jump pad is not working!  */
+  if (pthread_mutex_trylock (&mutex) != 0)
+    {
+      fail ();
+      return;
+    }
+
+  sleep (1);
+
+  if (pthread_mutex_unlock (&mutex) != 0)
+    {
+      fail ();
+      return;
+    }
+}
+
+/* Called from asm.  */
+static void __attribute__((used))
+func (void)
+{
+}
+
+static void *
+thread_function (void *arg)
+{
+  /* `set_point' is the label at which to set a fast tracepoint.  The
+     insn at the label must be large enough to fit a fast tracepoint
+     jump.  */
+  asm ("    .global " SYMBOL (set_point) "\n"
+       SYMBOL (set_point) ":\n"
+#if (defined __x86_64__ || defined __i386__)
+       "    call " SYMBOL (func) "\n"
+#elif (defined __aarch64__)
+       "    nop\n"
+#endif
+       );
+}
+
+static void
+end (void)
+{
+}
+
+int
+main (int argc, char *argv[], char *envp[])
+{
+  pthread_t threads[NUM_THREADS];
+  int i;
+
+  for (i = 0; i < NUM_THREADS; i++)
+    pthread_create (&threads[i], NULL, thread_function, NULL);
+
+  for (i = 0; i < NUM_THREADS; i++)
+    pthread_join (threads[i], NULL);
+
+  end ();
+
+  return 0;
+}