linux-kernel - Re: Is BIO_RW_FAILFAST really usable?

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Message-ID: <18263.12519.894142.682323@notabene.brown>
Date:	Thu, 6 Dec 2007 10:14:47 +1100
From:	Neil Brown <neilb@...e.de>
To:	Jens Axboe <jens.axboe@...cle.com>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: Is BIO_RW_FAILFAST really usable?

On Tuesday December 4, jens.axboe@...cle.com wrote:
> 
> Hrmpf. It looks like the SCSI layer is a little too trigger happy. Any
> chance you could try and trace where this happens?

in scsi_lib.c, in scsi_request_fn, near the top of the main
   while (!blk_queue_plugged(q)) {
loop:

		if (!scsi_dev_queue_ready(q, sdev)) {
			if ((req->cmd_flags & REQ_FAILFAST) &&
			    !(req->cmd_flags & REQ_PREEMPT)) {
				scsi_kill_request(req, q);
				continue;
			}
			break;
		}

If I remove the "if failfast and not preempt then kill" logic, my
problem goes away.

NeilBrown
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