job control: make task_clear_jobctl_pending() clear TRAPPING automatically

JOBCTL_TRAPPING indicates that ptracer is waiting for tracee to
(re)transit into TRACED.  task_clear_jobctl_pending() must be called
when either tracee enters TRACED or the transition is cancelled for
some reason.  The former is achieved by explicitly calling
task_clear_jobctl_pending() in ptrace_stop() and the latter by calling
it at the end of do_signal_stop().

Calling task_clear_jobctl_trapping() at the end of do_signal_stop()
limits the scope TRAPPING can be used and is fragile in that seemingly
unrelated changes to tracee's control flow can lead to stuck TRAPPING.

We already have task_clear_jobctl_pending() calls on those cancelling
events to clear JOBCTL_STOP_PENDING.  Cancellations can be handled by
making those call sites use JOBCTL_PENDING_MASK instead and updating
task_clear_jobctl_pending() such that task_clear_jobctl_trapping() is
called automatically if no stop/trap is pending.

This patch makes the above changes and removes the fallback
task_clear_jobctl_trapping() call from do_signal_stop().

Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Oleg Nesterov <oleg@redhat.com>

fs/exec.c

@@ -1772,7 +1772,7 @@ static int zap_process(struct task_struct *start, int exit_code)
 
 	t = start;
 	do {
-		task_clear_jobctl_pending(t, JOBCTL_STOP_PENDING);
+		task_clear_jobctl_pending(t, JOBCTL_PENDING_MASK);
 		if (t != current && t->mm) {
 			sigaddset(&t->pending.signal, SIGKILL);
 			signal_wake_up(t, 1);

kernel/signal.c

@@ -253,6 +253,9 @@ static void task_clear_jobctl_trapping(struct task_struct *task)
  * %JOBCTL_PENDING_MASK.  If %JOBCTL_STOP_PENDING is being cleared, other
  * STOP bits are cleared together.
  *
+ * If clearing of @mask leaves no stop or trap pending, this function calls
+ * task_clear_jobctl_trapping().
+ *
  * CONTEXT:
  * Must be called with @task->sighand->siglock held.
  */
@@ -264,6 +267,9 @@ void task_clear_jobctl_pending(struct task_struct *task, unsigned int mask)
 		mask |= JOBCTL_STOP_CONSUME | JOBCTL_STOP_DEQUEUED;
 
 	task->jobctl &= ~mask;
+
+	if (!(task->jobctl & JOBCTL_PENDING_MASK))
+		task_clear_jobctl_trapping(task);
 }
 
 /**
@@ -933,7 +939,7 @@ static void complete_signal(int sig, struct task_struct *p, int group)
 			signal->group_stop_count = 0;
 			t = p;
 			do {
-				task_clear_jobctl_pending(t, JOBCTL_STOP_PENDING);
+				task_clear_jobctl_pending(t, JOBCTL_PENDING_MASK);
 				sigaddset(&t->pending.signal, SIGKILL);
 				signal_wake_up(t, 1);
 			} while_each_thread(p, t);
@@ -1168,7 +1174,7 @@ int zap_other_threads(struct task_struct *p)
 	p->signal->group_stop_count = 0;
 
 	while_each_thread(p, t) {
-		task_clear_jobctl_pending(t, JOBCTL_STOP_PENDING);
+		task_clear_jobctl_pending(t, JOBCTL_PENDING_MASK);
 		count++;
 
 		/* Don't bother with already dead threads */
@@ -1964,9 +1970,6 @@ retry:
 		goto retry;
 	}
 
-	/* PTRACE_ATTACH might have raced with task killing, clear trapping */
-	task_clear_jobctl_trapping(current);
-
 	spin_unlock_irq(&current->sighand->siglock);
 
 	tracehook_finish_jctl();