Re: [PATCH scsi-misc-2.6 08/08] scsi: fix hot unplug sequence

[Tejun Heo]

 Hello, James and Jens.

On Thu, Mar 24, 2005 at 06:45:58PM -0600, James Bottomley wrote:
> On Wed, 2005-03-23 at 16:25 +0100, Jens Axboe wrote:
> > Let me guess, it is hanging in wait_for_completion()?
> 
> Yes, I have the trace now.  Why is curious.  This is the trace of the
> failure:
> 
> Mar 24 18:40:34 localhost kernel: usb 4-2: USB disconnect, address 3
> Mar 24 18:40:34 localhost kernel: sd 0:0:0:0: CMD c25c98b0 done, completing
> Mar 24 18:40:34 localhost kernel:  0:0:0:0: cmd c25c98b0 returning
> Mar 24 18:40:34 localhost kernel:  0:0:0:0: cmd c25c98b0 going out <6>Read Capacity (10) 25 00 00 00 00 00 00 00 00 00
> Mar 24 18:40:34 localhost kernel: scsi0 (0:0): rejecting I/O to dead device (req c25c98b0)
> Mar 24 18:40:34 localhost kernel: usb 4-2: new full speed USB device using uhci_hcd and address 4
> Mar 24 18:40:34 localhost kernel: scsi1 : SCSI emulation for USB Mass Storage devices
> Mar 24 18:40:34 localhost kernel:  1:0:0:0: cmd c1a1b4b0 going out <6>Inquiry 12 00 00 00 24 00
> 
> The problem occurs when the mid-layer rejects the I/O to the dead
> device.  Here it returns BLKPREP_KILL to the prep function, but after
> that we never get a completion back.

 I think I found the cause.  Special requests submitted using
scsi_do_req() never initializes ->end_io().  Normally, SCSI midlayer
terminates special requests inside the SCSI midlayer without passing
through the blkdev layer.  However, if a device is going away or taken
offline, blkdev layer gets to terminate special requests and, as
->end_io() is never set-up, nothing happens and the completion gets
lost.

 The following patch implements scsi_do_req_endio() and sets up
->end_io() and ->end_io_data before sending out special commands.
It's a quick fix & hacky.  I think the proper fix might be one of

 * Don't return BLKPREP_KILL in the prep_fn and always terminate
   special commands inside request_fn without using end_that_*
   functions.

 * I don't really know why the scsi_request/scsi_cmnd distincion
   is made (resource usage?), but, if it's a legacy thing, replace
   scsi_request with scsi_cmnd; then we can BLKPREP_KILL without using
   dummy scsi_cmnd.


 Signed-off-by: Tejun Heo <htejun@xxxxxxxxx>


# This is a BitKeeper generated diff -Nru style patch.
#
# ChangeSet
#   2005/03/25 11:57:25+09:00 tj@xxxxxxxxxxxxxx 
#   midlayer special command termination fix
# 
# drivers/scsi/scsi_lib.c
#   2005/03/25 11:57:17+09:00 tj@xxxxxxxxxxxxxx +30 -0
#   midlayer special command termination fix
# 
diff -Nru a/drivers/scsi/scsi_lib.c b/drivers/scsi/scsi_lib.c
--- a/drivers/scsi/scsi_lib.c	2005-03-25 11:59:28 +09:00
+++ b/drivers/scsi/scsi_lib.c	2005-03-25 11:59:28 +09:00
@@ -274,6 +274,33 @@
 }
 
 /*
+ * Special requests usually gets terminated inside scsi midlayer
+ * proper; however, they can be terminated by the blkdev layer when
+ * scsi_prep_fn() returns BLKPREP_KILL or scsi_request_fn() detects
+ * offline condition.  The following callback is invoked when the
+ * blkdev layer terminates a special request.  Emulate DID_NO_CONNECT.
+ */
+static void scsi_do_req_endio(struct request *rq)
+{
+	struct scsi_request *sreq = rq->end_io_data;
+	struct request_queue *q = sreq->sr_device->request_queue;
+	struct scsi_cmnd cmd;
+
+	memset(&cmd, 0, sizeof(cmd));
+	cmd.device = sreq->sr_device;
+	scsi_init_cmd_from_req(&cmd, sreq);
+	/* Our command is dummy, nullify back link. */
+	sreq->sr_command = NULL;
+
+	sreq->sr_result = cmd.result = DID_NO_CONNECT << 16;
+
+	/* The sreq->done() callback expects queue_lock to be unlocked. */
+	spin_unlock(q->queue_lock);
+	cmd.done(&cmd);
+	spin_lock(q->queue_lock);
+}
+
+/*
  * Function:    scsi_do_req
  *
  * Purpose:     Queue a SCSI request
@@ -326,6 +353,9 @@
 
 	if (sreq->sr_cmd_len == 0)
 		sreq->sr_cmd_len = COMMAND_SIZE(sreq->sr_cmnd[0]);
+
+	sreq->sr_request->end_io = scsi_do_req_endio;
+	sreq->sr_request->end_io_data = sreq;
 
 	/*
 	 * head injection *required* here otherwise quiesce won't work
-
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