thinkpad-acpi: kill hotkey_thread_mutex

hotkey_kthread() does try_to_freeze() under hotkey_thread_mutex.

We can simply kill this mutex, hotkey_poll_stop_sync() does not need to
serialize with hotkey_kthread().  When kthread_stop() returns the thread
is already dead, it called do_exit()->complete_vfork_done().
Reported-by: Artem Savkov <artem.savkov@gmail.com>
Reported-by: Maciej Rutecki <maciej.rutecki@gmail.com>
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Acked-by: Henrique de Moraes Holschuh <hmh@hmh.eng.br>
Cc: Matthew Garrett <matthew.garrett@nebula.com>
Cc: "Rafael J. Wysocki" <rjw@sisk.pl>
Reviewed-by: Mandeep Singh Baines <msb@chromium.org>
Cc: Aaron Lu <aaron.lu@intel.com>
Cc: Tejun Heo <tj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/platform/x86/thinkpad_acpi.c

@@ -1964,9 +1964,6 @@ struct tp_nvram_state {
 /* kthread for the hotkey poller */
 static struct task_struct *tpacpi_hotkey_task;
 
-/* Acquired while the poller kthread is running, use to sync start/stop */
-static struct mutex hotkey_thread_mutex;
-
 /*
  * Acquire mutex to write poller control variables as an
  * atomic block.
@@ -2462,8 +2459,6 @@ static int hotkey_kthread(void *data)
 	unsigned int poll_freq;
 	bool was_frozen;
 
-	mutex_lock(&hotkey_thread_mutex);
-
 	if (tpacpi_lifecycle == TPACPI_LIFE_EXITING)
 		goto exit;
 
@@ -2523,7 +2518,6 @@ static int hotkey_kthread(void *data)
 	}
 
 exit:
-	mutex_unlock(&hotkey_thread_mutex);
 	return 0;
 }
 
@@ -2533,9 +2527,6 @@ static void hotkey_poll_stop_sync(void)
 	if (tpacpi_hotkey_task) {
 		kthread_stop(tpacpi_hotkey_task);
 		tpacpi_hotkey_task = NULL;
-		mutex_lock(&hotkey_thread_mutex);
-		/* at this point, the thread did exit */
-		mutex_unlock(&hotkey_thread_mutex);
 	}
 }
 
@@ -3234,7 +3225,6 @@ static int __init hotkey_init(struct ibm_init_struct *iibm)
 	mutex_init(&hotkey_mutex);
 
 #ifdef CONFIG_THINKPAD_ACPI_HOTKEY_POLL
-	mutex_init(&hotkey_thread_mutex);
 	mutex_init(&hotkey_thread_data_mutex);
 #endif