Re: Question about Request Sense case in scsi_lib.c

[James Bottomley <James.Bottomley@SteelEye.com>]

On Thu, 2004-10-14 at 01:49, Lan Tran wrote:
> And it was due to the fact that when a bio is sent down to the
> mid-layer, it would come back with another bio chained to it, so when
> the original bio was retried, the  number of segments that were mapped
> (i.e. 2, one from each bio) did not match the value stored in
> cmd->use_sg (i.e. 1).

This sounds a bit unlikely, since the SCSI layer never deals with bios
per se, it merely maps a request (which is a collection of bios).  What
was the evidence that this was happening?

I can see the reverse being true: the scsi request is partially complete
when requeued, so some of the bios are fully complete and somehow this
causes a miscalculation in the segments.  However, the miscalculation
has to be that we undercount the number of needed request slots, and
this looks hard to do.

>  I still haven't figured out why chained bios
> from indepedent IO requests are returned from the mid-layer ... but
> may be a similar issue you're seeing here?

It sounds similar.  The problem seems to be in requeuing somehow.  I'm
going to dig out my old requeueing simulator and see if I can reproduce
it.

James