block: Invalidate the cache for a parent block-device if blkdev_issue_flush() was called for a partition

Executing a BLKFLSBUF-ioctl on a partition flushes the caches for that partition but reading data through the parent device will still return the old cached data.

The cache for the block-device is not synced if the block-device is kept open (due to a mounted partition, for example). Only when all users for the disk have exited, the cache for the disk is made consistent again.

Calling invalidate_bdev() on the parent block-device in case blkdev_issue_flush() was called for a partition fixes this.

The problem can be worked around by forcing the caches to be flushed with either

	# blockdev --flushbufs ${dev_disk}
or
	# echo 3 > /proc/sys/vm/drop_caches

---
 block/blk-flush.c |    3 +++
 1 files changed, 3 insertions(+), 0 deletions(-)

index 720ad60..e876f8e 100644
@@ -448,6 +448,9 @@ int blkdev_issue_flush(struct block_device *bdev, gfp_t gfp_mask,
 
 	if (!bio_flagged(bio, BIO_UPTODATE))
 		ret = -EIO;
+	else if (bdev != bdev->bd_contains)
+		/* invalidate parent block_device */
+		invalidate_bdev(bdev->bd_contains);
 
 	bio_put(bio);
 	return ret;

1.7.6.5

Cheers,
Bryn.