block: add request tracking
The block layer does not know about pending requests. This information is necessary for copy-on-read since overlapping requests must be serialized to prevent races that corrupt the image. The BlockDriverState gets a new tracked_request list field which contains all pending requests. Each request is a BdrvTrackedRequest record with sector_num, nb_sectors, and is_write fields. Note that request tracking is always enabled but hopefully this extra work is so small that it doesn't justify adding an enable/disable flag. Signed-off-by: NStefan Hajnoczi <stefanha@linux.vnet.ibm.com> Signed-off-by: NKevin Wolf <kwolf@redhat.com>
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