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[dm-devel] Re: [PATCH 27/28] blk_end_request: changing scsi mid-layer for bidi (take 3)



Hi Boaz,

On Tue, 04 Dec 2007 15:39:12 +0200, Boaz Harrosh <bharrosh panasas com> wrote:
> On Sat, Dec 01 2007 at 1:35 +0200, Kiyoshi Ueda <k-ueda ct jp nec com> wrote:
> > This patch converts bidi of scsi mid-layer to use blk_end_request().
> > 
> > rq->next_rq represents a pair of bidi requests.
> > (There are no other use of 'next_rq' of struct request.)
> > For both requests in the pair, end_that_request_chunk() should be
> > called before end_that_request_last() is called for one of them.
> > Since the calls to end_that_request_first()/chunk() and
> > end_that_request_last() are packaged into blk_end_request(),
> > the handling of next_rq completion has to be moved into
> > blk_end_request(), too.
> > 
> > Bidi sets its specific value to rq->data_len before the request is
> > completed so that upper-layer can read it.
> > This setting must be between end_that_request_chunk() and
> > end_that_request_last(), because rq->data_len may be used
> > in end_that_request_chunk() by blk_trace and so on.
> > To satisfy the requirement, use blk_end_request_callback() which
> > is added in PATCH 25 only for the tricky drivers.
> > 
> > If bidi didn't reuse rq->data_len and added new members to request
> > for the specific value, it could set before end_that_request_chunk()
> > and use the standard blk_end_request() like below.
> > 
> > void scsi_end_bidi_request(struct scsi_cmnd *cmd)
> > {
> > 	struct request *req = cmd->request;
> > 
> > 	rq->resid = scsi_out(cmd)->resid;
> > 	rq->next_rq->resid = scsi_in(cmd)->resid;
> > 
> > 	if (blk_end_request(req, 1, req->data_len))
> > 		BUG();
> > 
> > 	scsi_release_buffers(cmd);
> > 	scsi_next_command(cmd);
> > }
...
snip
...
>
> rq->data_len = scsi_out(cmd)->resid is Not Just a problem of bidi
> it is a General problem of scsi residual handling, and user code.
> 
> Even today before any bidi. at scsi_lib.c at scsi_io_completion()
> we do req->data_len = scsi_get_resid(cmd);
> ( or: req->data_len = cmd->resid; depends which version you look)
> And then call scsi_end_request() which calls __end_that_request_first/last
> So it is assumed even today that req->data_len is not touched by
> __end_that_request_first/last unless __end_that_request_first returned
> that there is more work to do and the command is resubmitted in which
> case the resid information is discarded.
> 
> So if the regular resid handling is acceptable - Set req->data_len
> before the call to __end_that_request_first/last, or blk_end_request()
> in your case, then here goes your second client of the _callback and
> it can be removed.
> But if it is found that req->data_len is touched and the resid information
> gets lost, than it should be fixed for the common uni-io case, by - for example
> - pass resid to the blk_end_request() function.
> (So in any way the _callback can go)

Thank you for the explanation of scsi's rq->data_len usage.
I see that scsi usually uses rq->data_len for cmd->resid.

I have investigated the possibility of setting data_len before
the call to blk_end_request.
But no matter whether data_len is touched or not, we need a callback
for bidi.  So I would like to go with the current patch.

I explained the reason and some details below.


As far as I can see, rq->data_len is just referenced
by blk_add_trace_rq() in __end_that_request_first(), not modified.
And I don't change any logic around there in the block-layer.
So there shouldn't be any critical problem for scsi residual handing.
(although I'm not sure that scsi expectes cmd->resid to be traced
 by blk_trace.)

Anyway, I see that it is no critical problem for bidi to set cmd->resid
to rq->data_len before blk_end_request() call.
But if I do that, blk_end_request() can't get the next_rq's size
to complete in its code below.

> +		/* Bidi request must be completed as a whole */
> +		if (blk_bidi_rq(rq) &&
> +		    __end_that_request_first(rq->next_rq, uptodate,
> +					     blk_rq_bytes(rq->next_rq)))
> +			return 1;

So I will have to move next_rq completion to bidi and use _callback()
anyway like the following.
---------------------------------------------------------------------
static int dummy_cb(struct request *rq)
{
	return 1;
}

void scsi_end_bidi_request(struct scsi_cmnd *cmd)
{
 	struct request *req = cmd->request;
	unsigned int dlen = req->data_len;
	unsigned int next_dlen = req->next_rq->data_len;
 
 	req->data_len = scsi_out(cmd)->resid;
 	req->next_rq->data_len = scsi_in(cmd)->resid;
 
	/* Complete only DATA of next_rq using _callback and dummy function */
	if (!blk_end_request_callback(req->next_rq, 1, next_dlen, dummy_cb))
		BUG();
 
	if (blk_end_request(req, 1, dlen))
		BUG();

	scsi_release_buffers(cmd);
	scsi_next_command(cmd);
}
---------------------------------------------------------------------

I prefer the current patch rather than the code like above,
since the code calls blk_end_request twice and looks trickier.
So I'd like to leave the patch but descriptions and comments in it.
Below is the patch, which updated only descriptions and comments.
If you still have some concerns on this, please let me know.



Subject: [PATCH 27/28] blk_end_request: changing scsi mid-layer for bidi (take 3)

This patch converts bidi of scsi mid-layer to use blk_end_request
interfaces.

rq->next_rq represents a pair of bidi requests.
(There are no other use of 'next_rq' of struct request.)
For both requests in the pair, end_that_request_chunk() should be
called before end_that_request_last() is called for one of them.
Since the calls to end_that_request_first()/chunk() and
end_that_request_last() are packaged into blk_end_request(),
the handling of next_rq completion has to be moved into
blk_end_request(), too.

Since blk_end_request() family don't have any argument for
the completion size of next_rq, they use next_rq->data_len for it.
So bidi has to set the resid after blk_end_request() completes
the data of next_rq.
To satisfy the requirement, bidi uses blk_end_request_callback(),
which is added in PATCH 25 only for the tricky drivers.

Signed-off-by: Kiyoshi Ueda <k-ueda ct jp nec com>
Signed-off-by: Jun'ichi Nomura <j-nomura ce jp nec com>
---
 block/ll_rw_blk.c       |   18 +++++++++++++
 drivers/scsi/scsi_lib.c |   62 +++++++++++++++++++++++-------------------------
 2 files changed, 48 insertions(+), 32 deletions(-)

Index: 2.6.24-rc3-mm2/drivers/scsi/scsi_lib.c
===================================================================
--- 2.6.24-rc3-mm2.orig/drivers/scsi/scsi_lib.c
+++ 2.6.24-rc3-mm2/drivers/scsi/scsi_lib.c
@@ -629,28 +629,6 @@ void scsi_run_host_queues(struct Scsi_Ho
 		scsi_run_queue(sdev->request_queue);
 }
 
-static void scsi_finalize_request(struct scsi_cmnd *cmd, int uptodate)
-{
-	struct request_queue *q = cmd->device->request_queue;
-	struct request *req = cmd->request;
-	unsigned long flags;
-
-	add_disk_randomness(req->rq_disk);
-
-	spin_lock_irqsave(q->queue_lock, flags);
-	if (blk_rq_tagged(req))
-		blk_queue_end_tag(q, req);
-
-	end_that_request_last(req, uptodate);
-	spin_unlock_irqrestore(q->queue_lock, flags);
-
-	/*
-	 * This will goose the queue request function at the end, so we don't
-	 * need to worry about launching another command.
-	 */
-	scsi_next_command(cmd);
-}
-
 /*
  * Function:    scsi_end_request()
  *
@@ -921,6 +899,20 @@ void scsi_release_buffers(struct scsi_cm
 EXPORT_SYMBOL(scsi_release_buffers);
 
 /*
+ * Called from blk_end_request_callback() after all DATA in rq and its next_rq
+ * are completed before rq is completed/freed.
+ */
+static int scsi_end_bidi_request_cb(struct request *rq)
+{
+	struct scsi_cmnd *cmd = rq->special;
+
+	rq->data_len = scsi_out(cmd)->resid;
+	rq->next_rq->data_len = scsi_in(cmd)->resid;
+
+	return 0;
+}
+
+/*
  * Bidi commands Must be complete as a whole, both sides at once.
  * If part of the bytes were written and lld returned
  * scsi_in()->resid and/or scsi_out()->resid this information will be left
@@ -931,22 +923,28 @@ void scsi_end_bidi_request(struct scsi_c
 {
 	struct request *req = cmd->request;
 
-	end_that_request_chunk(req, 1, req->data_len);
-	req->data_len = scsi_out(cmd)->resid;
-
-	end_that_request_chunk(req->next_rq, 1, req->next_rq->data_len);
-	req->next_rq->data_len = scsi_in(cmd)->resid;
-
-	scsi_release_buffers(cmd);
-
 	/*
 	 *FIXME: If ll_rw_blk.c is changed to also put_request(req->next_rq)
-	 *       in end_that_request_last() then this WARN_ON must be removed.
+	 *       in blk_end_request() then this WARN_ON must be removed.
 	 *       for now, upper-driver must have registered an end_io.
 	 */
 	WARN_ON(!req->end_io);
 
-	scsi_finalize_request(cmd, 1);
+	/*
+	 * blk_end_request() family take care of data completion of next_rq.
+	 * blk_end_request() family use next_rq->data_len for 
+	 * the completion data size of next_rq.
+	 * So resid can't be set before the data completion of next_rq
+	 * in blk_end_request().
+	 * To resolve that, use the callback feature of blk_end_request().
+	 */
+	if (blk_end_request_callback(req, 1, req->data_len,
+				     scsi_end_bidi_request_cb))
+		/* req has not been completed */
+		BUG();
+
+	scsi_release_buffers(cmd);
+	scsi_next_command(cmd);
 }
 
 /*
Index: 2.6.24-rc3-mm2/block/ll_rw_blk.c
===================================================================
--- 2.6.24-rc3-mm2.orig/block/ll_rw_blk.c
+++ 2.6.24-rc3-mm2/block/ll_rw_blk.c
@@ -3817,6 +3817,12 @@ int blk_end_request(struct request *rq, 
 	if (blk_fs_request(rq) || blk_pc_request(rq)) {
 		if (__end_that_request_first(rq, uptodate, nr_bytes))
 			return 1;
+
+		/* Bidi request must be completed as a whole */
+		if (blk_bidi_rq(rq) &&
+		    __end_that_request_first(rq->next_rq, uptodate,
+					     blk_rq_bytes(rq->next_rq)))
+			return 1;
 	}
 
 	add_disk_randomness(rq->rq_disk);
@@ -3840,6 +3846,12 @@ int __blk_end_request(struct request *rq
 	if (blk_fs_request(rq) || blk_pc_request(rq)) {
 		if (__end_that_request_first(rq, uptodate, nr_bytes))
 			return 1;
+
+		/* Bidi request must be completed as a whole */
+		if (blk_bidi_rq(rq) &&
+		    __end_that_request_first(rq->next_rq, uptodate,
+					     blk_rq_bytes(rq->next_rq)))
+			return 1;
 	}
 
 	add_disk_randomness(rq->rq_disk);
@@ -3884,6 +3896,12 @@ int blk_end_request_callback(struct requ
 	if (blk_fs_request(rq) || blk_pc_request(rq)) {
 		if (__end_that_request_first(rq, uptodate, nr_bytes))
 			return 1;
+
+		/* Bidi request must be completed as a whole */
+		if (blk_bidi_rq(rq) &&
+		    __end_that_request_first(rq->next_rq, uptodate,
+					     blk_rq_bytes(rq->next_rq)))
+			return 1;
 	}
 
 	/* Special feature for tricky drivers */

Thanks,
Kiyoshi Ueda


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