driver core: synchronize device shutdown

A patch series to make .shutdown execute asynchronously.  Some drivers's
shutdown can take a lot of time.  The patches can help save some shutdown
time.  The patches use Arjan's async API.


This patch:

synchronize all tasks submitted by .shutdown
Signed-off-by: Shaohua Li <shaohua.li@intel.com>
Cc: Arjan van de Ven <arjan@infradead.org>
Cc: Dmitry Torokhov <dtor@mail.ru>
Cc: James Bottomley <James.Bottomley@HansenPartnership.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

drivers/base/core.c

@@ -22,6 +22,7 @@
 #include <linux/kallsyms.h>
 #include <linux/semaphore.h>
 #include <linux/mutex.h>
+#include <linux/async.h>
 
 #include "base.h"
 #include "power/power.h"
@@ -1665,4 +1666,5 @@ void device_shutdown(void)
 	kobject_put(sysfs_dev_char_kobj);
 	kobject_put(sysfs_dev_block_kobj);
 	kobject_put(dev_kobj);
+	async_synchronize_full();
 }