Subject: Re: [PATCH] scsi_lib.c: continue after MEDIUM_ERROR
From: James Bottomley <James.Bottomley@HansenPartnership.com>
To: Mark Lord <liml@rtr.ca>
Cc: Ric Wheeler <ric@emc.com>, "Eric D. Mudama" <edmudama@gmail.com>,
       linux-kernel@vger.kernel.org,
       IDE/ATA development list <linux-ide@vger.kernel.org>,
       linux-scsi <linux-scsi@vger.kernel.org>, dougg@torque.net
In-Reply-To: <45C0B211.2030305@rtr.ca>
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Date: Wed, 31 Jan 2007 09:24:30 -0600
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On Wed, 2007-01-31 at 10:13 -0500, Mark Lord wrote:
> James Bottomley wrote:
> >
> > For the MD case, this is what REQ_FAILFAST is for.

> I cannot find where SCSI honours that flag.  James?

Er, it's in scsi_error.c:scsi_decide_disposition():

      maybe_retry:

	/* we requeue for retry because the error was retryable, and
	 * the request was not marked fast fail.  Note that above,
	 * even if the request is marked fast fail, we still requeue
	 * for queue congestion conditions (QUEUE_FULL or BUSY) */
	if ((++scmd->retries) <= scmd->allowed
	    && !blk_noretry_request(scmd->request)) {
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
		return NEEDS_RETRY;
	} else {
		/*
		 * no more retries - report this one back to upper level.
		 */
		return SUCCESS;
	}

> And for that matter, even when I patch SCSI so that it *does* honour it,
> I don't actually see the flag making it into the SCSI layer from above.
> 
> And I don't see where/how the block layer takes care when considering
> merge FAILFAST/READA requests with non FAILFAST/READA requests.
> To me, it looks perfectly happy to add non-FAILFAST/READA bios
> to a FAILFAST request, risking data loss if a lower-layer decides
> to honour the FAILFAST/READA flags.
> 
> So it's a pretty Good Thing(tm) that SCSI doesn't currently honour it. ;)

James


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