md: protect against NULL reference when waiting to start a raid10.

When we fail to start a raid10 for some reason, we call
md_unregister_thread to kill the thread that was created.

Unfortunately md_thread() will then make one call into the handler
(raid10d) even though md_wakeup_thread has not been called.  This is
not safe and as md_unregister_thread is called after mddev->private
has been set to NULL, it will definitely cause a NULL dereference.

So fix this at both ends:
 - md_thread should only call the handler if THREAD_WAKEUP has been
   set.
 - raid10 should call md_unregister_thread before setting things
   to NULL just like all the other raid modules do.

This is applicable to 2.6.35 and later.

Cc: stable@kernel.org
Reported-by: "Citizen" <citizen_lee@thecus.com>
Signed-off-by: NeilBrown <neilb@suse.de>

drivers/md/md.c

@@ -6044,9 +6044,8 @@ static int md_thread(void * arg)
 			 || kthread_should_stop(),
 			 thread->timeout);
 
-		clear_bit(THREAD_WAKEUP, &thread->flags);
-
-		thread->run(thread->mddev);
+		if (test_and_clear_bit(THREAD_WAKEUP, &thread->flags))
+			thread->run(thread->mddev);
 	}
 
 	return 0;

drivers/md/raid10.c

@@ -2397,13 +2397,13 @@ static int run(mddev_t *mddev)
 	return 0;
 
 out_free_conf:
+	md_unregister_thread(mddev->thread);
 	if (conf->r10bio_pool)
 		mempool_destroy(conf->r10bio_pool);
 	safe_put_page(conf->tmppage);
 	kfree(conf->mirrors);
 	kfree(conf);
 	mddev->private = NULL;
-	md_unregister_thread(mddev->thread);
 out:
 	return -EIO;
 }