gdb.threads/step-over-thread-exit.exp improvements

This commit makes the following improvements to
gdb.threads/step-over-thread-exit.exp:

- Add a third axis to stepping over the breakpoint with displaced vs
  inline stepping -- also test with no breakpoint at all.

- Check that when GDB reports "Command aborted, thread exited.", the
  selected thread is the thread that exited.  This is always true
  currently on GNU/Linux by coincidence, but a similar testcase on AMD
  GPU exposed a problem here.  Better make the testcase catch any
  potential regression.

- Fixes a race that Simon ran into with GDBserver testing.

    (gdb) next
    [New Thread 2143071.2143438]

    Thread 3 "step-over-threa" hit Breakpoint 2, 0x000055555555524e in my_exit_syscall () at .../testsuite/lib/my-syscalls.S:74
    74      SYSCALL (my_exit, __NR_exit)
    (gdb) FAIL: gdb.threads/step-over-thread-exit.exp: displaced-stepping=auto: non-stop=on: target-non-stop=on: schedlock=off: cmd=next: ns_stop_all=0: command aborts when thread exits

  I was not able to reproduce it, but I believe that what happens is
  the following:

  Once we continue, the thread 2 exits, and the main thread thus
  unblocks from its pthread_join, and spawns a new thread.  That new
  thread may hit the breakpoint at my_exit_syscall very quickly.  GDB
  could then see/process that breakpoint event before the thread exit
  event for the thread we care about, which would result in the
  failure seen above.

  The fix here is to not loop and start a new thread at all in the
  scenario where the race can happen.  We only need to loop and spawn
  new threads when testing with "cmd=continue" and schedlock off, in
  which case GDB doesn't abort the command when the thread exits.

Approved-By: Simon Marchi <simon.marchi@efficios.com>
Change-Id: I90c95c32f00630a3f682b1541c23aff52451f9b6

2 files changed, 119 insertions, 24 deletions

diff --git a/gdb/testsuite/gdb.threads/step-over-thread-exit.c b/gdb/testsuite/gdb.threads/step-over-thread-exit.c
index 878e592..218f003 100644
--- a/gdb/testsuite/gdb.threads/step-over-thread-exit.c
+++ b/gdb/testsuite/gdb.threads/step-over-thread-exit.c
@@ -18,6 +18,7 @@
 #include <pthread.h>
 #include <assert.h>
 #include <stdlib.h>
+#include <unistd.h>
 #include "../lib/my-syscalls.h"
 
 static void *
@@ -30,13 +31,19 @@ thread_func (void *arg)
   abort ();
 }
 
+/* Number of threads we'll create.  */
+int n_threads = 100;
+
 int
-main (void)
+main (int argc, char **argv)
 {
   int i;
 
-  /* Spawn and join a thread, 100 times.  */
-  for (i = 0; i < 100; i++)
+  if (argc > 1)
+    n_threads = atoi (argv[1]);
+
+  /* Spawn and join a thread, N_THREADS times.  */
+  for (i = 0; i < n_threads; i++)
     {
       pthread_t thread;
       int ret;
@@ -48,5 +55,8 @@ main (void)
       assert (ret == 0);
     }
 
+  /* Some time to make sure that GDB processes the thread exit event
+     before the whole-process exit.  */
+  sleep (3);
   return 0;
 }
diff --git a/gdb/testsuite/gdb.threads/step-over-thread-exit.exp b/gdb/testsuite/gdb.threads/step-over-thread-exit.exp
index 615bd83..32f64ce 100644
--- a/gdb/testsuite/gdb.threads/step-over-thread-exit.exp
+++ b/gdb/testsuite/gdb.threads/step-over-thread-exit.exp
@@ -25,11 +25,29 @@ if { [build_executable "failed to prepare" $testfile \
     return
 }
 
-# Each argument is a different testing axis, most of them obvious.
+# Test stepping/continuing at an exit syscall instruction.
+#
+# Each argument is a different testing axis.
+#
+# STEP_OVER_MODE can be one of:
+#
+#   - none: don't put a breakpoint on the exit syscall instruction.
+#
+#   - inline: put a breakpoint on the exit syscall instruction, and
+#     use in-line stepping to step over it (disable
+#     displaced-stepping).
+#
+#   - displaced: same, but use displaced stepping.
+#
+# SCHEDLOCK can be "on" or "off".
+#
+# CMD is the GDB command to run when at the exit syscall instruction.
+#
 # NS_STOP_ALL is only used if testing "set non-stop on", and indicates
 # whether to have GDB explicitly stop all threads before continuing to
 # thread exit.
-proc test {displaced-stepping non-stop target-non-stop schedlock cmd ns_stop_all} {
+#
+proc test {step_over_mode non-stop target-non-stop schedlock cmd ns_stop_all} {
     if {${non-stop} == "off" && $ns_stop_all} {
 	error "invalid arguments"
     }
@@ -40,23 +58,29 @@ proc test {displaced-stepping non-stop target-non-stop schedlock cmd ns_stop_all
 	clean_restart $::binfile
     }
 
-    gdb_test_no_output "set displaced-stepping ${displaced-stepping}"
-
-    if { ![runto_main] } {
-	return
+    if { $step_over_mode == "none" } {
+	# Nothing to do.
+    } elseif { $step_over_mode == "inline" } {
+	gdb_test_no_output "set displaced-stepping off"
+    } elseif { $step_over_mode == "displaced" } {
+	gdb_test_no_output "set displaced-stepping on"
+    } else {
+	error "Invalid step_over_mode value: $step_over_mode"
     }
 
-    gdb_breakpoint "my_exit_syscall"
-
     if {$schedlock
 	|| (${non-stop} == "on" && $ns_stop_all)} {
-	gdb_test "continue" \
-	    "Thread 2 .*hit Breakpoint $::decimal.* my_exit_syscall .*" \
-	    "continue until syscall"
+
+	gdb_test_no_output "set args 1"
+
+	if { ![runto my_exit_syscall] } {
+	    return
+	}
 
 	if {${non-stop} == "on"} {
 	    # The test only spawns one thread at a time, so this just
-	    # stops the main thread.
+	    # stops the main thread.  IOW, we only need to wait for
+	    # one stop.
 	    gdb_test_multiple "interrupt -a" "" {
 		-re "$::gdb_prompt " {
 		    gdb_test_multiple "" $gdb_test_name {
@@ -66,12 +90,19 @@ proc test {displaced-stepping non-stop target-non-stop schedlock cmd ns_stop_all
 		    }
 		}
 	    }
-	}
 
-	gdb_test "thread 2" "Switching to thread 2 .*"
+	    gdb_test "thread 2" "Switching to thread 2 .*"
+	}
 
 	gdb_test_no_output "set scheduler-locking ${schedlock}"
 
+	# If testing a step-over is requested, leave the breakpoint at
+	# the current instruction to force a step-over; otherwise,
+	# remove it.
+	if { $step_over_mode == "none" } {
+	    delete_breakpoints
+	}
+
 	if {$cmd == "continue"} {
 	    gdb_test "continue" \
 		"No unwaited-for children left." \
@@ -84,9 +115,50 @@ proc test {displaced-stepping non-stop target-non-stop schedlock cmd ns_stop_all
 	    }
 	}
     } else {
+	# Schedlock is off here.
+	#
+	# With "continue" and no scheduler-locking, GDB doesn't stop
+	# with "Command aborted, thread exited." when the thread
+	# exits, it just lets the inferior continue running freely.
+	# So we test that we can move past the thread exit, and that
+	# other threads can be freely scheduled.  We do that by
+	# spawning another thread as soon as the first exit.  We test
+	# that a number of times.  This should also exercise GDB's
+	# handling of inline or displaced step-overs, that GDB handles
+	# the related resource accounting correctly when the stepping
+	# thread exits, etc.
+	#
+	# With "continue" and $step_over_mode == "none" however, after
+	# the first my_exit_syscall breakpoint hit, we will remove the
+	# breakpoint, so no other thread would ever hit it again.  So
+	# might as well just test one thread.
+	#
+	# With step/next, GDB aborts the execution command with
+	# "Command aborted, thread exited." when the stepping thread
+	# exits.  If we let the main spawn another thread as soon as
+	# the first exits, it would be possible for that new thread to
+	# hit the exit syscall insn breakpoint quickly enough that it
+	# would be reported to be user before the first thread exit
+	# would be, which would confuse testing.  To avoid that, we
+	# only spawn one thread, too.
+	#
+	if {$cmd != "continue" || $step_over_mode == "none"} {
+	    set n_threads 1
+	} else {
+	    set n_threads 100
+	}
+
+	gdb_test_no_output "set args $n_threads"
+
+	if { ![runto_main] } {
+	    return
+	}
+
+	gdb_breakpoint "my_exit_syscall"
+
 	gdb_test_no_output "set scheduler-locking ${schedlock}"
 
-	if {$cmd != "continue"} {
+	if {$cmd != "continue" || $step_over_mode == "none"} {
 	    set thread "<unknown>"
 	    gdb_test_multiple "continue" "" {
 		-re -wrap "Thread ($::decimal) .*hit Breakpoint $::decimal.* my_exit_syscall .*" {
@@ -98,10 +170,23 @@ proc test {displaced-stepping non-stop target-non-stop schedlock cmd ns_stop_all
 		    "switch to event thread"
 	    }
 
-	    gdb_test_multiple $cmd "command aborts when thread exits" {
-		-re "Command aborted, thread exited\\.\r\n$::gdb_prompt " {
-		    pass $gdb_test_name
+	    # If testing a step-over is requested, leave the breakpoint at
+	    # the current instruction to force a step-over; otherwise,
+	    # remove it.
+	    if { $step_over_mode == "none" } {
+		delete_breakpoints
+	    }
+
+	    if {$cmd == "continue"} {
+		gdb_continue_to_end "continue to end" "continue" 1
+	    } else {
+		gdb_test_multiple $cmd "command aborts when thread exits" {
+		    -re "Command aborted, thread exited\\.\r\n$::gdb_prompt " {
+			pass $gdb_test_name
+		    }
 		}
+		gdb_test "p \$_thread == $thread" "= 1" \
+		    "selected thread didn't change"
 	    }
 	} else {
 	    for { set i 0 } { $i < 100 } { incr i } {
@@ -130,7 +215,7 @@ proc test {displaced-stepping non-stop target-non-stop schedlock cmd ns_stop_all
     }
 }
 
-foreach_with_prefix displaced-stepping {off auto} {
+foreach_with_prefix step_over_mode {none inline displaced} {
     foreach_with_prefix non-stop {off on} {
 	foreach_with_prefix target-non-stop {off on} {
 	    if {${non-stop} == "on" && ${target-non-stop} == "off"} {
@@ -142,11 +227,11 @@ foreach_with_prefix displaced-stepping {off auto} {
 		foreach_with_prefix cmd {"next" "continue"} {
 		    if {${non-stop} == "on"} {
 			foreach_with_prefix ns_stop_all {0 1} {
-			    test ${displaced-stepping} ${non-stop} ${target-non-stop} \
+			    test ${step_over_mode} ${non-stop} ${target-non-stop} \
 				${schedlock} ${cmd} ${ns_stop_all}
 			}
 		    } else {
-			test ${displaced-stepping} ${non-stop} ${target-non-stop} ${schedlock} ${cmd} 0
+			test ${step_over_mode} ${non-stop} ${target-non-stop} ${schedlock} ${cmd} 0
 		    }
 		}
 	    }