Re: Possible bug in scsi_lib.c:scsi_req_map_sg()

[Mike Christie <michaelc@cs.wisc.edu>]

James Bottomley wrote:
> On Sun, 2007-03-04 at 09:43 -0600, Mike Christie wrote:
>> There are trying to do a scsi level multipath driver for RDAC support.
>> And they are trying to do something similar to request based multipath
>> (route requests instead of bios but in their case they are routing
>> scsi_cmnds instead of requests).
> 
> I guessed it might be something like that ... 
> 
> The SCSI driver layer wasn't designed to stack like that ... hence the
> problems of setup and teardown of the sg list.  Wouldn't it just be
> easier to make request based multi-path work?  Unfortunately, you can't
> avoid the request system because what you want to do is resubmit an I/O
> without having the mid-layer execute the prepare function (which is
> where it's mapped) so you need the REQ_DONTPREP flag.
> 

I think they get around this and other request settings that need
resetting by using scsi_execute_async. They will take the command, data
direction and buffer fields from the original scsi_cmnd, then pass those
on to scsi_ececute_async which would allocate a new request and then as
you know that new request gets sent to the scsi layer and looks like a
brand new request. So I misspoke above. It might be better to say they
are using it for routing what the other multipath layers would call
cloned commands.