[SCSI] Fix refcount breakage with 'echo "1" > scan' when target already present

Spotted by: Dan Aloni <[e-mail]>

The problem is there's inconsistent locking semantic usage of
scsi_alloc_target(). Two callers assume the target comes back with
reference unincremented and the third assumes its incremented. Fix by
always making the reference incremented on return. Also fix path in
target alloc that could consistently increment the parent lock.
Finally document scsi_alloc_target() so its callers know what the
expectations are.

Signed-off-by: James Bottomley <[e-mail]>

drivers/scsi/scsi_scan.c

···
 	return found_starget;
 }
 
+/**
+ * scsi_alloc_target - allocate a new or find an existing target
+ * @parent:	parent of the target (need not be a scsi host)
+ * @channel:	target channel number (zero if no channels)
+ * @id:		target id number
+ *
+ * Return an existing target if one exists, provided it hasn't already
+ * gone into STARGET_DEL state, otherwise allocate a new target.
+ *
+ * The target is returned with an incremented reference, so the caller
+ * is responsible for both reaping and doing a last put
+ */
 static struct scsi_target *scsi_alloc_target(struct device *parent,
 					     int channel, uint id)
 {
···
 			return NULL;
 		}
 	}
+	get_device(dev);
 
 	return starget;
 
  found:
 	found_target->reap_ref++;
 	spin_unlock_irqrestore(shost->host_lock, flags);
-	put_device(parent);
 	if (found_target->state != STARGET_DEL) {
+		put_device(parent);
 		kfree(starget);
 		return found_target;
 	}
···
 	if (!starget)
 		return ERR_PTR(-ENOMEM);
 
-	get_device(&starget->dev);
 	mutex_lock(&shost->scan_mutex);
 	if (scsi_host_scan_allowed(shost))
 		scsi_probe_and_add_lun(starget, lun, NULL, &sdev, 1, hostdata);
···
 	if (!starget)
 		return;
 
-	get_device(&starget->dev);
 	if (lun != SCAN_WILD_CARD) {
 		/*
 		 * Scan for a specific host/chan/id/lun.
···
 	if (sdev) {
 		sdev->sdev_gendev.parent = get_device(&starget->dev);
 		sdev->borken = 0;
-	}
+	} else
+		scsi_target_reap(starget);
 	put_device(&starget->dev);
  out:
 	mutex_unlock(&shost->scan_mutex);