From mboxrd@z Thu Jan 1 00:00:00 1970 From: Christof Schmitt Subject: Re: scsi_cmnd data_buffer checksum Date: Thu, 9 Sep 2010 10:00:28 +0200 Message-ID: <20100909080028.GA4880@schmichrtp.mainz.de.ibm.com> References: <35352.83946.qm@web32402.mail.mud.yahoo.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Received: from mtagate3.de.ibm.com ([195.212.17.163]:59323 "EHLO mtagate3.de.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751474Ab0IIIAa (ORCPT ); Thu, 9 Sep 2010 04:00:30 -0400 Content-Disposition: inline In-Reply-To: <35352.83946.qm@web32402.mail.mud.yahoo.com> Sender: linux-scsi-owner@vger.kernel.org List-Id: linux-scsi@vger.kernel.org To: Anil kumar Cc: linux-kernel@vger.kernel.org, linux-scsi@vger.kernel.org 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