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

This reverts commit 23196f2e5f5d810578a772785807dcdc2b9fdce9.

Now that struct kthread is kmalloc'ed and not longer on the task stack
there is no need anymore to pin the stack.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Chunming Zhou <David1.Zhou@amd.com>
Cc: Roman Pen <roman.penyaev@profitbricks.com>
Cc: Petr Mladek <pmladek@suse.com>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Tejun Heo <tj@kernel.org>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Link: http://lkml.kernel.org/r/20161129175100.GA5333@redhat.com
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

kernel/kthread.c

@@ -88,7 +88,7 @@ void free_kthread_struct(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) && try_get_task_stack(k))
+	if (likely(vfork))
 		return __to_kthread(vfork);
 	return NULL;
 }
@@ -473,10 +473,8 @@ 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);
 
@@ -505,7 +503,6 @@ int kthread_park(struct task_struct *k)
 				wait_for_completion(&kthread->parked);
 			}
 		}
-		put_task_stack(k);
 		ret = 0;
 	}
 	return ret;
@@ -541,7 +538,6 @@ 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);