IB/uverbs: Avoid a crash on device hot remove

Wait until all users have closed their device context before allowing
device unregistration to complete. This prevents a crash caused by
referring to stale data structures.

A better solution would be to have a way to revoke contexts rather
than waiting for userspace to close the context, but that's a much
bigger change that will have to wait. For now let's at least avoid
the crash.

Signed-off-by: Jack Morgenstein <[e-mail]>
Signed-off-by: Michael S. Tsirkin <[e-mail]>
Signed-off-by: Roland Dreier <[e-mail]>

drivers/infiniband/core/uverbs.h

···
 #include <linux/kref.h>
 #include <linux/idr.h>
 #include <linux/mutex.h>
+#include <linux/completion.h>
 
 #include <rdma/ib_verbs.h>
 #include <rdma/ib_user_verbs.h>
···
 
 struct ib_uverbs_device {
 	struct kref				ref;
+	struct completion			comp;
 	int					devnum;
 	struct cdev			       *dev;
 	struct class_device		       *class_dev;

drivers/infiniband/core/uverbs_main.c

···
 	struct ib_uverbs_device *dev =
 		container_of(ref, struct ib_uverbs_device, ref);
 
-	kfree(dev);
+	complete(&dev->comp);
 }
 
 void ib_uverbs_release_ucq(struct ib_uverbs_file *file,
···
 		return;
 
 	kref_init(&uverbs_dev->ref);
+	init_completion(&uverbs_dev->comp);
 
 	spin_lock(&map_lock);
 	uverbs_dev->devnum = find_first_zero_bit(dev_map, IB_UVERBS_MAX_DEVICES);
···
 
 err:
 	kref_put(&uverbs_dev->ref, ib_uverbs_release_dev);
+	wait_for_completion(&uverbs_dev->comp);
+	kfree(uverbs_dev);
 	return;
 }
 
···
 	spin_unlock(&map_lock);
 
 	clear_bit(uverbs_dev->devnum, dev_map);
+
 	kref_put(&uverbs_dev->ref, ib_uverbs_release_dev);
+	wait_for_completion(&uverbs_dev->comp);
+	kfree(uverbs_dev);
 }
 
 static int uverbs_event_get_sb(struct file_system_type *fs_type, int flags,