[Xen-devel] [PATCH 11/16] blkfront: Lock blkfront_info when closing

[Jeremy Fitzhardinge <jeremy@xxxxxxxx>]

From: Daniel Stodden <daniel.stodden@xxxxxxxxxx>

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@xxxxxxxxxx>
Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@xxxxxxxxxx>
---
 drivers/block/xen-blkfront.c |   61 +++++++++++++++++++++++++++--------------
 1 files changed, 40 insertions(+), 21 deletions(-)

diff --git a/drivers/block/xen-blkfront.c b/drivers/block/xen-blkfront.c
index 7b03b48..1046a58 100644
--- a/drivers/block/xen-blkfront.c
+++ b/drivers/block/xen-blkfront.c
@@ -78,6 +78,7 @@ 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;
        }
 }
-- 
1.7.1.1


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