[Bug 15565] New: SCSI Generic queueing completes commands in reverse order

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http://bugzilla.kernel.org/show_bug.cgi?id=15565

           Summary: SCSI Generic queueing completes commands in reverse
                    order
           Product: IO/Storage
           Version: 2.5
    Kernel Version: 2.6.18-2.6.32
          Platform: All
        OS/Version: Linux
              Tree: Mainline
            Status: NEW
          Severity: normal
          Priority: P1
         Component: SCSI
        AssignedTo: linux-scsi@vger.kernel.org
        ReportedBy: mh-linux-kernel@loup.net
        Regression: No


I've noticed after queueing the first command, subsequent commands
appear to be executed and complete in reverse order.  The SCSI Generic
HOWTO says "By default, read() will return the oldest completed
request that is queued up."

This could also be a performance defect if it's what's really
happening since it isn't desirable behavior if, for example, ios are
typically ordered by lba and issued one at a time by kernel to a non
queueing block device.

This reverse order behavior is trivial to reproduce; just queue 16
concurrent INQUIRY commands.  The following are typical results I get
from initially queueing 16 READ_10 commands.

   Completion Command #  hdr.driver_duration (us)
   Order
   1          22         14979
   2          20         14981
   3          19         14982
   4          17         14984
   5          16         14985
   6          15         14986
   7          14         14988
   8          12         14990
   9          11         14991
   10         10         14992

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