blkfront: Lock blkfront_info when closing

The bdev .open/.release fops race against backend switches to Closing,
handled by the XenBus thread.

The original code attempted to serialize block device holders and
xenbus only via bd_mutex. This is insufficient, the info->bd pointer
may already be stale (or null) while xenbus tries to bump up the
refcount.

Protect blkfront_info with a dedicated mutex.

Signed-off-by: Daniel Stodden <daniel.stodden@citrix.com>
Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>

1 files changed, 40 insertions, 21 deletions

diff --git a/drivers/block/xen-blkfront.c b/drivers/block/xen-blkfront.c
index d4cb7fd82b4c..1e406f0331e7 100644
--- a/drivers/block/xen-blkfront.c
+++ b/drivers/block/xen-blkfront.c
@@ -79,6 +79,7 @@ static const struct block_device_operations xlvbd_block_fops;
  */
 struct blkfront_info
 {
+        struct mutex mutex;
         struct xenbus_device *xbdev;
         struct gendisk *gd;
         int vdevice;
@@ -804,7 +805,6 @@ again:
         return err;
 }
 
-
 /**
  * Entry point to this code when a new device is created.  Allocate the basic
  * structures and the ring buffer for communication with the backend, and
@@ -836,6 +836,7 @@ static int blkfront_probe(struct xenbus_device *dev,
                 return -ENOMEM;
         }
 
+        mutex_init(&info->mutex);
         info->xbdev = dev;
         info->vdevice = vdevice;
         info->connected = BLKIF_STATE_DISCONNECTED;
@@ -951,6 +952,43 @@ static int blkfront_resume(struct xenbus_device *dev)
         return err;
 }
 
+static void
+blkfront_closing(struct blkfront_info *info)
+{
+        struct xenbus_device *xbdev = info->xbdev;
+        struct block_device *bdev = NULL;
+
+        mutex_lock(&info->mutex);
+
+        if (xbdev->state == XenbusStateClosing) {
+                mutex_unlock(&info->mutex);
+                return;
+        }
+
+        if (info->gd)
+                bdev = bdget_disk(info->gd, 0);
+
+        mutex_unlock(&info->mutex);
+
+        if (!bdev) {
+                xenbus_frontend_closed(xbdev);
+                return;
+        }
+
+        mutex_lock(&bdev->bd_mutex);
+
+        if (info->users) {
+                xenbus_dev_error(xbdev, -EBUSY,
+                                 "Device in use; refusing to close");
+                xenbus_switch_state(xbdev, XenbusStateClosing);
+        } else {
+                xlvbd_release_gendisk(info);
+                xenbus_frontend_closed(xbdev);
+        }
+
+        mutex_unlock(&bdev->bd_mutex);
+        bdput(bdev);
+}
 
 /*
  * Invoked when the backend is finally 'ready' (and has told produced
@@ -1034,7 +1072,6 @@ static void blkback_changed(struct xenbus_device *dev,
                             enum xenbus_state backend_state)
 {
         struct blkfront_info *info = dev_get_drvdata(&dev->dev);
-        struct block_device *bd;
 
         dev_dbg(&dev->dev, "blkfront:blkback_changed to state %d.\n", backend_state);
 
@@ -1051,25 +1088,7 @@ static void blkback_changed(struct xenbus_device *dev,
                 break;
 
         case XenbusStateClosing:
-                if (info->gd == NULL) {
-                        xenbus_frontend_closed(dev);
-                        break;
-                }
-                bd = bdget_disk(info->gd, 0);
-                if (bd == NULL)
-                        xenbus_dev_fatal(dev, -ENODEV, "bdget failed");
-
-                mutex_lock(&bd->bd_mutex);
-                if (info->users > 0)
-                        xenbus_dev_error(dev, -EBUSY,
-                                         "Device in use; refusing to close");
-                else {
-                        xlvbd_release_gendisk(info);
-                        xenbus_frontend_closed(info->xbdev);
-                }
-
-                mutex_unlock(&bd->bd_mutex);
-                bdput(bd);
+                blkfront_closing(info);
                 break;
         }
 }