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>

1 files changed, 8 insertions, 5 deletions

diff --git a/kernel/signal.c b/kernel/signal.c
index 288d952fa3b8..637a171b65b6 100644
--- a/kernel/signal.c
+++ b/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();