do_signal_stop() can call tracehook_notify_jctl() before decrementing

->group_stop_count and setting TASK_STOPPED/SIGNAL_STOP_STOPPED.

This way the tracing hooks can drop and reacquire the siglock freely and
do any blocking hooks without potential SIGCONT races.

With this patch TASK_STOPPED/SIGNAL_STOP_STOPPED is set only when we know
for sure we are going to schedule() after unlock(siglock).
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Acked-by: Roland McGrath <roland@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

kernel/signal.c

@@ -1664,16 +1664,9 @@ void ptrace_notify(int exit_code)
 static int do_signal_stop(int signr)
 {
 	struct signal_struct *sig = current->signal;
-	int stop_count;
 	int notify;
 
-	if (sig->group_stop_count > 0) {
-		/*
-		 * There is a group stop in progress.  We don't need to
-		 * start another one.
-		 */
-		stop_count = --sig->group_stop_count;
-	} else {
+	if (!sig->group_stop_count) {
 		struct task_struct *t;
 
 		if (!likely(sig->flags & SIGNAL_STOP_DEQUEUED) ||
@@ -1685,7 +1678,7 @@ static int do_signal_stop(int signr)
 		 */
 		sig->group_exit_code = signr;
 
-		stop_count = 0;
+		sig->group_stop_count = 1;
 		for (t = next_thread(current); t != current; t = next_thread(t))
 			/*
 			 * Setting state to TASK_STOPPED for a group
@@ -1694,25 +1687,28 @@ static int do_signal_stop(int signr)
 			 */
 			if (!(t->flags & PF_EXITING) &&
 			    !task_is_stopped_or_traced(t)) {
-				stop_count++;
+				sig->group_stop_count++;
 				signal_wake_up(t, 0);
 			}
-		sig->group_stop_count = stop_count;
 	}
-
-	if (stop_count == 0)
-		sig->flags = SIGNAL_STOP_STOPPED;
-	current->exit_code = sig->group_exit_code;
-	__set_current_state(TASK_STOPPED);
-
 	/*
 	 * If there are no other threads in the group, or if there is
-	 * a group stop in progress and we are the last to stop,
-	 * report to the parent.  When ptraced, every thread reports itself.
+	 * a group stop in progress and we are the last to stop, report
+	 * to the parent.  When ptraced, every thread reports itself.
 	 */
-	notify = tracehook_notify_jctl(stop_count == 0 ? CLD_STOPPED : 0,
-				       CLD_STOPPED);
-
+	notify = sig->group_stop_count == 1 ? CLD_STOPPED : 0;
+	notify = tracehook_notify_jctl(notify, CLD_STOPPED);
+	/*
+	 * tracehook_notify_jctl() can drop and reacquire siglock, so
+	 * we keep ->group_stop_count != 0 before the call. If SIGCONT
+	 * or SIGKILL comes in between ->group_stop_count == 0.
+	 */
+	if (sig->group_stop_count) {
+		if (!--sig->group_stop_count)
+			sig->flags = SIGNAL_STOP_STOPPED;
+		current->exit_code = sig->group_exit_code;
+		__set_current_state(TASK_STOPPED);
+	}
 	spin_unlock_irq(&current->sighand->siglock);
 
 	if (notify) {