handle_stop_signal: unify partial/full stop handling

Now that handle_stop_signal() doesn't drop ->siglock, we can't see both
->group_stop_count && SIGNAL_STOP_STOPPED.  Merge two "if" branches.

As Roland pointed out, we never actually needed 2 do_notify_parent_cldstop()
calls.

Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>
Cc: Roland McGrath <roland@redhat.com>
Cc: Jiri Kosina <jkosina@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

kernel/signal.c

@@ -585,33 +585,16 @@ static void handle_stop_signal(int sig, struct task_struct *p)
 			t = next_thread(t);
 		} while (t != p);
 	} else if (sig == SIGCONT) {
+		unsigned int why;
 		/*
 		 * Remove all stop signals from all queues,
 		 * and wake all threads.
 		 */
-		if (unlikely(p->signal->group_stop_count > 0)) {
-			/*
-			 * There was a group stop in progress.  We'll
-			 * pretend it finished before we got here.  We are
-			 * obliged to report it to the parent: if the
-			 * SIGSTOP happened "after" this SIGCONT, then it
-			 * would have cleared this pending SIGCONT.  If it
-			 * happened "before" this SIGCONT, then the parent
-			 * got the SIGCHLD about the stop finishing before
-			 * the continue happened.  We do the notification
-			 * now, and it's as if the stop had finished and
-			 * the SIGCHLD was pending on entry to this kill.
-			 */
-			p->signal->group_stop_count = 0;
-			p->signal->flags = SIGNAL_STOP_CONTINUED |
-						SIGNAL_CLD_STOPPED;
-		}
 		rm_from_queue(SIG_KERNEL_STOP_MASK, &p->signal->shared_pending);
 		t = p;
 		do {
 			unsigned int state;
 			rm_from_queue(SIG_KERNEL_STOP_MASK, &t->pending);
-			
 			/*
 			 * If there is a handler for SIGCONT, we must make
 			 * sure that no thread returns to user mode before
@@ -621,7 +604,7 @@ static void handle_stop_signal(int sig, struct task_struct *p)
 			 * running the handler.  With the TIF_SIGPENDING
 			 * flag set, the thread will pause and acquire the
 			 * siglock that we hold now and until we've queued
-			 * the pending signal. 
+			 * the pending signal.
 			 *
 			 * Wake up the stopped thread _after_ setting
 			 * TIF_SIGPENDING
@@ -636,13 +619,23 @@ static void handle_stop_signal(int sig, struct task_struct *p)
 			t = next_thread(t);
 		} while (t != p);
 
-		if (p->signal->flags & SIGNAL_STOP_STOPPED) {
-			/*
-			 * We were in fact stopped, and are now continued.
-			 * Notify the parent with CLD_CONTINUED.
-			 */
-			p->signal->flags = SIGNAL_STOP_CONTINUED |
-						SIGNAL_CLD_CONTINUED;
+		/*
+		 * Notify the parent with CLD_CONTINUED if we were stopped.
+		 *
+		 * If we were in the middle of a group stop, we pretend it
+		 * was already finished, and then continued. Since SIGCHLD
+		 * doesn't queue we report only CLD_STOPPED, as if the next
+		 * CLD_CONTINUED was dropped.
+		 */
+		why = 0;
+		if (p->signal->flags & SIGNAL_STOP_STOPPED)
+			why |= SIGNAL_CLD_CONTINUED;
+		else if (p->signal->group_stop_count)
+			why |= SIGNAL_CLD_STOPPED;
+
+		if (why) {
+			p->signal->flags = why | SIGNAL_STOP_CONTINUED;
+			p->signal->group_stop_count = 0;
 			p->signal->group_exit_code = 0;
 		} else {
 			/*