block: use the passed in @bdev when claiming if partno is zero

6b4517a791 (block: implement bd_claiming and claiming block)
introduced claiming block to support O_EXCL blkdev opens properly.

bd_start_claiming() looks up the part 0 bdev and starts claiming
block.  The function assumed that there is only one part 0 bdev and
always used bdget_disk(disk, 0) to look it up; unfortunately, this
isn't true for some drivers (floppy) which use multiple block devices
to denote different operating parameters for the same physical device.
There can be multiple part 0 bdev's for the same device number.

This incorrect assumption caused the wrong bdev to be used during
claiming leading to unbalanced bd_holders as reported in the following
bug.

  https://bugzilla.kernel.org/show_bug.cgi?id=28522

This patch updates bd_start_claiming() such that it uses the bdev
specified as argument if its partno is zero.

Note that this means that different bdev's can be used for the same
device and O_EXCL check can be effectively bypassed.  It has always
been broken that way and floppy is fortunately on its way out.  Leave
that breakage alone.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reported-by: Alex Villacis Lasso <avillaci@ceibo.fiec.espol.edu.ec>
Tested-by: Alex Villacis Lasso <avillaci@ceibo.fiec.espol.edu.ec>
Cc: stable@kernel.org	# >= v2.6.36
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>

1 files changed, 13 insertions, 1 deletions

diff --git a/fs/block_dev.c b/fs/block_dev.c
index 1a2421f908f0..610e8e0b04b8 100644
--- a/fs/block_dev.c
+++ b/fs/block_dev.c
@@ -762,7 +762,19 @@ static struct block_device *bd_start_claiming(struct block_device *bdev,
         if (!disk)
                 return ERR_PTR(-ENXIO);
 
-        whole = bdget_disk(disk, 0);
+        /*
+         * Normally, @bdev should equal what's returned from bdget_disk()
+         * if partno is 0; however, some drivers (floppy) use multiple
+         * bdev's for the same physical device and @bdev may be one of the
+         * aliases.  Keep @bdev if partno is 0.  This means claimer
+         * tracking is broken for those devices but it has always been that
+         * way.
+         */
+        if (partno)
+                whole = bdget_disk(disk, 0);
+        else
+                whole = bdgrab(bdev);
+
         module_put(disk->fops->owner);
         put_disk(disk);
         if (!whole)