blkfront: Fix blkfront backend switch race (bdev open)

We need not mind if users grab a late handle on a closing disk. We
probably even should not. But we have to make sure it's not a dead
one already

Let the bdev deal with a gendisk deleted under its feet. Takes the
info mutex to decide a race against backend closing.

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

1 files changed, 24 insertions, 7 deletions

diff --git a/drivers/block/xen-blkfront.c b/drivers/block/xen-blkfront.c
index 1e406f0331e7..763a315712cc 100644
--- a/drivers/block/xen-blkfront.c
+++ b/drivers/block/xen-blkfront.c
@@ -1118,16 +1118,33 @@ static int blkfront_is_ready(struct xenbus_device *dev)
 
 static int blkif_open(struct block_device *bdev, fmode_t mode)
 {
-        struct blkfront_info *info = bdev->bd_disk->private_data;
-
-        if (!info->xbdev)
-                return -ENODEV;
+        struct gendisk *disk = bdev->bd_disk;
+        struct blkfront_info *info;
+        int err = 0;
 
         lock_kernel();
-        info->users++;
-        unlock_kernel();
 
-        return 0;
+        info = disk->private_data;
+        if (!info) {
+                /* xbdev gone */
+                err = -ERESTARTSYS;
+                goto out;
+        }
+
+        mutex_lock(&info->mutex);
+
+        if (!info->gd)
+                /* xbdev is closed */
+                err = -ERESTARTSYS;
+
+        mutex_unlock(&info->mutex);
+
+        if (!err)
+                ++info->users;
+
+        unlock_kernel();
+out:
+        return err;
 }
 
 static int blkif_release(struct gendisk *disk, fmode_t mode)