This changes tracehook_notify_jctl() so it's called with the siglock held,

and changes its argument and return value definition.  These clean-ups
make it a better fit for what new tracing hooks need to check.

Tracing needs the siglock here, held from the time TASK_STOPPED was set,
to avoid potential SIGCONT races if it wants to allow any blocking in its
tracing hooks.

This also folds the finish_stop() function into its caller
do_signal_stop().  The function is short, called only once and only
unconditionally.  It aids readability to fold it in.
Signed-off-by: Roland McGrath <roland@redhat.com>
Cc: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

include/linux/tracehook.h

 /*
  * Tracing hooks
  *
- * Copyright (C) 2008 Red Hat, Inc.  All rights reserved.
+ * Copyright (C) 2008-2009 Red Hat, Inc.  All rights reserved.
  *
  * This copyrighted material is made available to anyone wishing to use,
  * modify, copy, or redistribute it subject to the terms and conditions
@@ -463,22 +463,29 @@ static inline int tracehook_get_signal(struct task_struct *task,
 
 /**
  * tracehook_notify_jctl - report about job control stop/continue
- * @notify:		nonzero if this is the last thread in the group to stop
+ * @notify:		zero, %CLD_STOPPED or %CLD_CONTINUED
  * @why:		%CLD_STOPPED or %CLD_CONTINUED
  *
  * This is called when we might call do_notify_parent_cldstop().
- * It's called when about to stop for job control; we are already in
- * %TASK_STOPPED state, about to call schedule().  It's also called when
- * a delayed %CLD_STOPPED or %CLD_CONTINUED report is ready to be made.
  *
- * Return nonzero to generate a %SIGCHLD with @why, which is
- * normal if @notify is nonzero.
+ * @notify is zero if we would not ordinarily send a %SIGCHLD,
+ * or is the %CLD_STOPPED or %CLD_CONTINUED .si_code for %SIGCHLD.
  *
- * Called with no locks held.
+ * @why is %CLD_STOPPED when about to stop for job control;
+ * we are already in %TASK_STOPPED state, about to call schedule().
+ * It might also be that we have just exited (check %PF_EXITING),
+ * but need to report that a group-wide stop is complete.
+ *
+ * @why is %CLD_CONTINUED when waking up after job control stop and
+ * ready to make a delayed @notify report.
+ *
+ * Return the %CLD_* value for %SIGCHLD, or zero to generate no signal.
+ *
+ * Called with the siglock held.
  */
 static inline int tracehook_notify_jctl(int notify, int why)
 {
-	return notify || (current->ptrace & PT_PTRACED);
+	return notify ?: (current->ptrace & PT_PTRACED) ? why : 0;
 }
 
 #define DEATH_REAP			-1

kernel/signal.c

@@ -705,7 +705,7 @@ static int prepare_signal(int sig, struct task_struct *p, int from_ancestor_ns)
 
 		if (why) {
 			/*
-			 * The first thread which returns from finish_stop()
+			 * The first thread which returns from do_signal_stop()
 			 * will take ->siglock, notice SIGNAL_CLD_MASK, and
 			 * notify its parent. See get_signal_to_deliver().
 			 */
@@ -1655,29 +1655,6 @@ void ptrace_notify(int exit_code)
 	spin_unlock_irq(&current->sighand->siglock);
 }
 
-static void
-finish_stop(int stop_count)
-{
-	/*
-	 * 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.
-	 */
-	if (tracehook_notify_jctl(stop_count == 0, CLD_STOPPED)) {
-		read_lock(&tasklist_lock);
-		do_notify_parent_cldstop(current, CLD_STOPPED);
-		read_unlock(&tasklist_lock);
-	}
-
-	do {
-		schedule();
-	} while (try_to_freeze());
-	/*
-	 * Now we don't run again until continued.
-	 */
-	current->exit_code = 0;
-}
-
 /*
  * This performs the stopping for SIGSTOP and other stop signals.
  * We have to stop all threads in the thread group.
@@ -1688,6 +1665,7 @@ static int do_signal_stop(int signr)
 {
 	struct signal_struct *sig = current->signal;
 	int stop_count;
+	int notify;
 
 	if (sig->group_stop_count > 0) {
 		/*
@@ -1727,8 +1705,30 @@ static int do_signal_stop(int signr)
 	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.
+	 */
+	notify = tracehook_notify_jctl(stop_count == 0 ? CLD_STOPPED : 0,
+				       CLD_STOPPED);
+
 	spin_unlock_irq(&current->sighand->siglock);
-	finish_stop(stop_count);
+
+	if (notify) {
+		read_lock(&tasklist_lock);
+		do_notify_parent_cldstop(current, notify);
+		read_unlock(&tasklist_lock);
+	}
+
+	do {
+		schedule();
+	} while (try_to_freeze());
+	/*
+	 * Now we don't run again until continued.
+	 */
+	current->exit_code = 0;
+
 	return 1;
 }
 
@@ -1797,14 +1797,15 @@ relock:
 		int why = (signal->flags & SIGNAL_STOP_CONTINUED)
 				? CLD_CONTINUED : CLD_STOPPED;
 		signal->flags &= ~SIGNAL_CLD_MASK;
-		spin_unlock_irq(&sighand->siglock);
 
-		if (unlikely(!tracehook_notify_jctl(1, why)))
-			goto relock;
+		why = tracehook_notify_jctl(why, CLD_CONTINUED);
+		spin_unlock_irq(&sighand->siglock);
 
+		if (why) {
 			read_lock(&tasklist_lock);
 			do_notify_parent_cldstop(current->group_leader, why);
 			read_unlock(&tasklist_lock);
+		}
 		goto relock;
 	}
 
@@ -1969,14 +1970,14 @@ void exit_signals(struct task_struct *tsk)
 	if (unlikely(tsk->signal->group_stop_count) &&
 			!--tsk->signal->group_stop_count) {
 		tsk->signal->flags = SIGNAL_STOP_STOPPED;
-		group_stop = 1;
+		group_stop = tracehook_notify_jctl(CLD_STOPPED, CLD_STOPPED);
 	}
 out:
 	spin_unlock_irq(&tsk->sighand->siglock);
 
-	if (unlikely(group_stop) && tracehook_notify_jctl(1, CLD_STOPPED)) {
+	if (unlikely(group_stop)) {
 		read_lock(&tasklist_lock);
-		do_notify_parent_cldstop(tsk, CLD_STOPPED);
+		do_notify_parent_cldstop(tsk, group_stop);
 		read_unlock(&tasklist_lock);
 	}
 }