uninterruptible CLONE_VFORK (Was: oom: Make coredump interruptible)

[Oleg Nesterov <oleg@redhat.com>]

On 06/13, Oleg Nesterov wrote:
>
> On 06/13, KOSAKI Motohiro wrote:
> >
> > But, again, I have no objection to your patch. because I really hope to
> > fix coredump vs oom issue.
>
> Yes, I think this is important.

Oh. And another problem, vfork() is not interruptible too. This means
that the user can hide the memory hog from oom-killer. But let's forget
about oom.

Roland, any reason it should be uninterruptible? This doesn't look good
in any case. Perhaps the pseudo-patch below makes sense?

Oleg.

--- x/kernel/fork.c
+++ x/kernel/fork.c
@@ -1359,6 +1359,26 @@ struct task_struct * __cpuinit fork_idle
 	return task;
 }
 
+// ---------------------------------------------------
+// THIS SHOULD BE USED BY mm_release/coredump_wait/etc
+// ---------------------------------------------------
+void complete_vfork_done(struct task_struct *tsk)
+{
+	struct completion *vfork = xchg(tsk->vfork_done, NULL);
+	if (vfork)
+		complete(vfork);
+}
+
+static wait_for_vfork_done(struct task_struct *child, struct completion *vfork)
+{
+	if (!wait_for_completion_killable(vfork))
+		return;
+	if (xchg(child->vfork_done, NULL) != NULL)
+		return;
+	// the child has already read ->vfork_done and it should wake us up
+	wait_for_completion(vfork);
+}
+
 /*
  *  Ok, this is the main fork-routine.
  *
@@ -1433,6 +1453,7 @@ long do_fork(unsigned long clone_flags,
 		if (clone_flags & CLONE_VFORK) {
 			p->vfork_done = &vfork;
 			init_completion(&vfork);
+			get_task_struct(p);
 		}
 
 		audit_finish_fork(p);
@@ -1462,7 +1483,8 @@ long do_fork(unsigned long clone_flags,
 
 		if (clone_flags & CLONE_VFORK) {
 			freezer_do_not_count();
-			wait_for_completion(&vfork);
+			wait_for_vfork_done(p, &vfork);
+			put_task_struct(p),
 			freezer_count();
 			tracehook_report_vfork_done(p, nr);
 		}