On Wed, Sep 08, 2010 at 08:36:32PM -0700, Anil kumar wrote: > > I am writing a checksum calculation of scsi_cmnd data buffer in the driver. > > I calculate the checksum of the scsi_cmd data buffer(request_buffer) in driver queuecommand. > > Now when the command is completed from the hardware and before driver sends it back to mid-layer, I calculate the checksum again of the same scsi_cmd data_buffer again. > > Sometimes the checksums don't match. I mean somehow looks like OS changed the scsi_cmd data_buffer(request_buffer) in the meantime when driver is working on the command. > I print the address of the scsi_cmd data_buffer (virtual address) and its same and the contents of the buffer is also same during both the calculations. > > Can this happen? Yes. While a write I/O is being processed in Linux or in flight, the data buffers can change. This causes problems for other checksums as well; i ran into this when looking at the DIF/DIX checksums for SCSI commands. At the moment, this problem can be avoided e.g. by running only direct I/O on the xfs filesystem. There have been discussions about this, a short summary is here, look for "stable pages": http://lwn.net/Articles/399148/ Christof -- To unsubscribe from this list: send the line "unsubscribe linux-scsi" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
