kthread: Pin the stack via try_get_task_stack()/put_task_stack() in to_live_kthread() function

get_task_struct(tsk) no longer pins tsk->stack so all users of
to_live_kthread() should do try_get_task_stack/put_task_stack to protect
"struct kthread" which lives on kthread's stack.

TODO: Kill to_live_kthread(), perhaps we can even kill "struct kthread" too,
and rework kthread_stop(), it can use task_work_add() to sync with the exiting
kernel thread.

Message-Id: <20160629180357.GA7178@redhat.com>
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Andy Lutomirski <luto@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Jann Horn <jann@thejh.net>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/cb9b16bbc19d4aea4507ab0552e4644c1211d130.1474003868.git.luto@kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>

kernel/kthread.c

@@ -64,7 +64,7 @@ static inline struct kthread *to_kthread(struct task_struct *k)
 static struct kthread *to_live_kthread(struct task_struct *k)
 {
 	struct completion *vfork = ACCESS_ONCE(k->vfork_done);
-	if (likely(vfork))
+	if (likely(vfork) && try_get_task_stack(k))
 		return __to_kthread(vfork);
 	return NULL;
 }
@@ -425,8 +425,10 @@ void kthread_unpark(struct task_struct *k)
 {
 	struct kthread *kthread = to_live_kthread(k);
 
-	if (kthread)
+	if (kthread) {
 		__kthread_unpark(k, kthread);
+		put_task_stack(k);
+	}
 }
 EXPORT_SYMBOL_GPL(kthread_unpark);
 
@@ -455,6 +457,7 @@ int kthread_park(struct task_struct *k)
 				wait_for_completion(&kthread->parked);
 			}
 		}
+		put_task_stack(k);
 		ret = 0;
 	}
 	return ret;
@@ -490,6 +493,7 @@ int kthread_stop(struct task_struct *k)
 		__kthread_unpark(k, kthread);
 		wake_up_process(k);
 		wait_for_completion(&kthread->exited);
+		put_task_stack(k);
 	}
 	ret = k->exit_code;
 	put_task_struct(k);