Step over exit with reinsert breakpoints

This patch fixes a GDBserver crash when one thread is stepping over
a syscall instruction which is exit.  Step-over isn't finished due
to the exit, but GDBserver doesn't clean up the state of step-over,
so in the wait next time, GDBserver will wait on step_over_bkpt,
which is already exited, and GDBserver crashes because
'requested_child' is NULL.  See gdbserver logs below,

Need step over [LWP 14858]? yes, found breakpoint at 0x2aaaaad91307^M
proceed_all_lwps: found thread 14858 needing a step-over^M
Starting step-over on LWP 14858.  Stopping all threads^M
>>>> entering void stop_all_lwps(int, lwp_info*)
....
<<<< exiting void stop_all_lwps(int, lwp_info*)^M
Done stopping all threads for step-over.^M
pc is 0x2aaaaad91307^M
Writing 0f to 0x2aaaaad91307 in process 14858^M
Could not find fast tracepoint jump at 0x2aaaaad91307 in list (uninserting).^M
  pending reinsert at 0x2aaaaad91307^M
  step from pc 0x2aaaaad91307^M
Resuming lwp 14858 (step, signal 0, stop not expected)^M

 # Start step-over for LWP 14858

>>>> entering ptid_t linux_wait_1(ptid_t, target_waitstatus*, int)
....
LLFE: 14858 exited.
...
<<<< exiting ptid_t linux_wait_1(ptid_t, target_waitstatus*, int)

  # LWP 14858 exited
.....
>>>> entering ptid_t linux_wait_1(ptid_t, target_waitstatus*, int)^M
linux_wait_1: [<all threads>]^M
step_over_bkpt set [LWP 14858.14858], doing a blocking wait

  # but step_over_bkpt is still LWP 14858, which is wrong

The fix is to finish step-over if it is ongoing, and unsuspend other
threads.  Without the fix in linux-low.c, GDBserver will crash in
with running gdb.base/step-over-exit.exp.

gdb/gdbserver:

2016-06-17  Yao Qi  <yao.qi@linaro.org>

	* linux-low.c (unsuspend_all_lwps): Declare.
	(linux_low_filter_event): If thread exited, call finish_step_over.
	If step-over is finished, unsuspend other threads.

gdb/testsuite:

2016-06-17  Yao Qi  <yao.qi@linaro.org>

	* gdb.base/step-over-exit.c: New.
	* gdb.base/step-over-exit.exp: New.

3 files changed, 182 insertions, 0 deletions

diff --git a/gdb/testsuite/ChangeLog b/gdb/testsuite/ChangeLog
index 3dad273..fbdcd2b 100644
--- a/gdb/testsuite/ChangeLog
+++ b/gdb/testsuite/ChangeLog
@@ -1,3 +1,8 @@
+2016-06-17  Yao Qi  <yao.qi@linaro.org>
+
+	* gdb.base/step-over-exit.c: New.
+	* gdb.base/step-over-exit.exp: New.
+
 2016-06-17  Yan-Ting Lin  <currygt52@gmail.com>
 
 	* gdb.base/float.exp: Add target check for nds32*-*-*.
diff --git a/gdb/testsuite/gdb.base/step-over-exit.c b/gdb/testsuite/gdb.base/step-over-exit.c
new file mode 100644
index 0000000..fd0de71
--- /dev/null
+++ b/gdb/testsuite/gdb.base/step-over-exit.c
@@ -0,0 +1,50 @@
+/* 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 <stdlib.h>
+#include <unistd.h>
+#include <string.h>
+
+static void
+marker (void)
+{}
+
+int
+main (void)
+{
+  int  pid;
+
+  pid = fork ();
+  if (pid == 0) /* child */
+    {
+      _exit (0);
+    }
+  else
+    {
+    }
+
+  pid = fork ();
+  if (pid == 0) /* child */
+    {
+      marker ();
+      _exit (0);
+    }
+  else
+    {
+      marker ();
+    }
+}
diff --git a/gdb/testsuite/gdb.base/step-over-exit.exp b/gdb/testsuite/gdb.base/step-over-exit.exp
new file mode 100644
index 0000000..9f4c826
--- /dev/null
+++ b/gdb/testsuite/gdb.base/step-over-exit.exp
@@ -0,0 +1,127 @@
+#   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/>.
+
+standard_testfile
+
+# Test a thread is doing step-over a syscall instruction which is exit,
+# and GDBserver should cleanup its state of step-over properly.
+
+set syscall_insn ""
+
+# Define the syscall instruction for each target.
+
+if { [istarget "i\[34567\]86-*-linux*"] || [istarget "x86_64-*-linux*"] } {
+    set syscall_insn "\[ \t\](int|syscall|sysenter)\[ \t\]"
+} elseif { [istarget "aarch64*-*-linux*"] || [istarget "arm*-*-linux*"] } {
+    set syscall_insn "\[ \t\](swi|svc)\[ \t\]"
+} else {
+    unsupported "unknown syscall instruction"
+    return -1
+}
+
+if {[prepare_for_testing $testfile.exp $testfile $srcfile debug]} {
+    untested $testfile.exp
+    return -1
+}
+
+# Start with a fresh gdb.
+clean_restart ${testfile}
+if ![runto_main] {
+    fail "Can't run to main"
+    return -1
+}
+
+gdb_test "set follow-fork-mode child"
+gdb_test "set detach-on-fork off"
+
+# Step 1, find the syscall instruction address.
+
+gdb_test "break _exit" "Breakpoint $decimal at .*"
+
+# Hit the breakpoint on _exit.  The address of syscall insn is recorded.
+
+gdb_test "continue" \
+    "Continuing\\..*Breakpoint $decimal.*_exit \\(.*\\).*" \
+    "continue to exit"
+
+gdb_test "display/i \$pc" ".*"
+
+# Single step until we see a syscall insn or we reach the
+# upper bound of loop iterations.
+set msg "find syscall insn in exit"
+set steps 0
+set max_steps 1000
+gdb_test_multiple "stepi" $msg {
+    -re ".*$syscall_insn.*$gdb_prompt $" {
+	pass $msg
+    }
+    -re "x/i .*=>.*\r\n$gdb_prompt $" {
+	incr steps
+	if {$steps == $max_steps} {
+	    fail $msg
+	} else {
+	    send_gdb "stepi\n"
+	    exp_continue
+	}
+    }
+}
+
+if {$steps == $max_steps} {
+    return
+}
+
+# Remove the display
+gdb_test_no_output "delete display 1"
+
+set syscall_insn_addr [get_hexadecimal_valueof "\$pc" "0"]
+
+gdb_test "continue" "exited normally.*" "continue to end, first time"
+gdb_test "inferior 1" ".*Switching to inferior 1.*" \
+    "switch back to inferior 1, first time"
+
+delete_breakpoints
+
+gdb_test "break marker"
+
+gdb_test "continue" "Continuing\\..*Breakpoint $decimal, .*" \
+    "continue to marker, first time"
+
+# Step 2, create a breakpoint which evaluates false, and force it
+# evaluated on the target side.
+
+set test "set breakpoint condition-evaluation target"
+gdb_test_multiple $test $test {
+    -re "warning: Target does not support breakpoint condition evaluation.\r\nUsing host evaluation mode instead.\r\n$gdb_prompt $" {
+	# Target doesn't support breakpoint condition evaluation
+	# on its side, but it is no harm to run the test.
+    }
+    -re "^$test\r\n$gdb_prompt $" {
+    }
+}
+
+gdb_test "break \*$syscall_insn_addr if main == 0" \
+    "Breakpoint \[0-9\]* at .*"
+
+# Resume the child process, and the step-over is being done.
+
+gdb_test "continue" "exited normally.*" "continue to end, second time"
+gdb_test "inferior 1" ".*Switching to inferior 1.*" \
+    "switch back to inferior 1, second time"
+
+# Switch back to the parent process, continue to the marker to
+# test GDBserver's state is still correct.
+
+gdb_test "continue" "Continuing\\..*Breakpoint $decimal, .*" \
+    "continue to marker, second time"