kernel/stop_machine: partly revert "stop_machine: Use raw spinlocks"

With completion using swait and so rawlocks we don't need this anymore.
Further, bisect thinks this patch is responsible for:

|BUG: unable to handle kernel NULL pointer dereference at           (null)
|IP: [<ffffffff81082123>] sched_cpu_active+0x53/0x70
|PGD 0
|Oops: 0000 [#1] PREEMPT SMP DEBUG_PAGEALLOC
|Dumping ftrace buffer:
|   (ftrace buffer empty)
|Modules linked in:
|CPU: 1 PID: 0 Comm: swapper/1 Not tainted 4.4.1+ torvalds#330
|Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS Debian-1.8.2-1 04/01/2014
|task: ffff88013ae64b00 ti: ffff88013ae74000 task.ti: ffff88013ae74000
|RIP: 0010:[<ffffffff81082123>]  [<ffffffff81082123>] sched_cpu_active+0x53/0x70
|RSP: 0000:ffff88013ae77eb8  EFLAGS: 00010082
|RAX: 0000000000000001 RBX: ffffffff81c2cf20 RCX: 0000001050fb52fb
|RDX: 0000001050fb52fb RSI: 000000105117ca1e RDI: 00000000001c7723
|RBP: 0000000000000000 R08: 0000000000000000 R09: 0000000000000001
|R10: 0000000000000000 R11: 0000000000000001 R12: 00000000ffffffff
|R13: ffffffff81c2cee0 R14: 0000000000000000 R15: 0000000000000001
|FS:  0000000000000000(0000) GS:ffff88013b200000(0000) knlGS:0000000000000000
|CS:  0010 DS: 0000 ES: 0000 CR0: 000000008005003b
|CR2: 0000000000000000 CR3: 0000000001c09000 CR4: 00000000000006e0
|Stack:
| ffffffff810c446d ffff88013ae77f00 ffffffff8107d8dd 000000000000000a
| 0000000000000001 0000000000000000 0000000000000000 0000000000000000
| 0000000000000000 ffff88013ae77f10 ffffffff8107d90e ffff88013ae77f20
|Call Trace:
| [<ffffffff810c446d>] ? debug_lockdep_rcu_enabled+0x1d/0x20
| [<ffffffff8107d8dd>] ? notifier_call_chain+0x5d/0x80
| [<ffffffff8107d90e>] ? __raw_notifier_call_chain+0xe/0x10
| [<ffffffff810598a3>] ? cpu_notify+0x23/0x40
| [<ffffffff8105a7b8>] ? notify_cpu_starting+0x28/0x30

during hotplug. The rawlocks need to remain however.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>

kernel/stop_machine.c

@@ -30,7 +30,7 @@ struct cpu_stop_done {
  atomic_t nr_todo; /* nr left to execute */
  bool executed; /* actually executed? */
  int ret; /* collected return value */
- struct task_struct *waiter; /* woken when nr_todo reaches 0 */
+ struct completion completion; /* fired if nr_todo reaches 0 */
 };
 
 /* the actual stopper, one per every possible cpu, enabled on online cpus */
@@ -59,7 +59,7 @@ static void cpu_stop_init_done(struct cpu_stop_done *done, unsigned int nr_todo)
 {
  memset(done, 0, sizeof(*done));
  atomic_set(&done->nr_todo, nr_todo);
- done->waiter = current;
+ init_completion(&done->completion);
 }
 
 /* signal completion unless @done is NULL */
@@ -68,10 +68,8 @@ static void cpu_stop_signal_done(struct cpu_stop_done *done, bool executed)
  if (done) {
  if (executed)
  done->executed = true;
- if (atomic_dec_and_test(&done->nr_todo)) {
- wake_up_process(done->waiter);
- done->waiter = NULL;
- }
+ if (atomic_dec_and_test(&done->nr_todo))
+ complete(&done->completion);
  }
 }
 
@@ -96,22 +94,6 @@ static void cpu_stop_queue_work(unsigned int cpu, struct cpu_stop_work *work)
  raw_spin_unlock_irqrestore(&stopper->lock, flags);
 }
 
-static void wait_for_stop_done(struct cpu_stop_done *done)
-{
- set_current_state(TASK_UNINTERRUPTIBLE);
- while (atomic_read(&done->nr_todo)) {
- schedule();
- set_current_state(TASK_UNINTERRUPTIBLE);
- }
- /*
- * We need to wait until cpu_stop_signal_done() has cleared
- * done->waiter.
- */
- while (done->waiter)
- cpu_relax();
- set_current_state(TASK_RUNNING);
-}
-
 /**
  * stop_one_cpu - stop a cpu
  * @cpu: cpu to stop
@@ -143,7 +125,7 @@ int stop_one_cpu(unsigned int cpu, cpu_stop_fn_t fn, void *arg)
 
  cpu_stop_init_done(&done, 1);
  cpu_stop_queue_work(cpu, &work);
- wait_for_stop_done(&done);
+ wait_for_completion(&done.completion);
  return done.executed ? done.ret : -ENOENT;
 }
 
@@ -302,7 +284,7 @@ int stop_two_cpus(unsigned int cpu1, unsigned int cpu2, cpu_stop_fn_t fn, void *
 
  preempt_enable_nort();
 
- wait_for_stop_done(&done);
+ wait_for_completion(&done.completion);
 
  return done.executed ? done.ret : -ENOENT;
 }
@@ -364,7 +346,7 @@ static int __stop_cpus(const struct cpumask *cpumask,
 
  cpu_stop_init_done(&done, cpumask_weight(cpumask));
  queue_stop_cpus_work(cpumask, fn, arg, &done, false);
- wait_for_stop_done(&done);
+ wait_for_completion(&done.completion);
  return done.executed ? done.ret : -ENOENT;
 }
 
@@ -495,13 +477,7 @@ static void cpu_stopper_thread(unsigned int cpu)
  kallsyms_lookup((unsigned long)fn, NULL, NULL, NULL,
  ksym_buf), arg);
 
- /*
- * Make sure that the wakeup and setting done->waiter
- * to NULL is atomic.
- */
- local_irq_disable();
  cpu_stop_signal_done(done, true);
- local_irq_enable();
  goto repeat;
  }
 }
@@ -663,7 +639,7 @@ int stop_machine_from_inactive_cpu(cpu_stop_fn_t fn, void *data,
  ret = multi_cpu_stop(&msdata);
 
  /* Busy wait for completion. */
- while (atomic_read(&done.nr_todo))
+ while (!completion_done(&done.completion))
  cpu_relax();
 
  mutex_unlock(&stop_cpus_mutex);