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Subject[PATCH] Possible SCSI + block-layer bugs
Hi,

I've never seen these trigger, but they look theoretically possible.

When processing the completion of a SCSI request in a bottom-half,
__scsi_end_request() can find all the buffers associated with the request
haven't been completed (ie. leftovers).

One question is; can this ever happen?
If it can't then the code should be removed from __scsi_end_request(),
if it can happen then there appears to be a few problems;

The request is re-queued to the block layer via
scsi_queue_next_request(), which uses the "special" pointer in the request
structure to remember the Scsi_Cmnd associated with the request. The SCSI
request function is then called, but doesn't guarantee to immediately
process the re-queued request even though it was added at the head (say,
the queue has become plugged). This can trigger two possible bugs.

The first is that __scsi_end_request() doesn't decrement the
hard_nr_sectors count in the request. As the request is back on the
queue, it is possible for newly arriving buffer-heads to merge with the
heads already hanging off the request. This merging uses the
hard_nr_sectors when calculating both the merged hard_nr_sectors and
nr_sectors counts.
As the request is at the head, only back-merging can occur, but if
__scsi_end_request() triggers another uncompleted request to be re-queued,
it is possible to get front merging as well.

The merging of a re-queued request looks safe, except for the
hard_nr_sectors. This patch corrects the hard_nr_sectors accounting.


The second bug is from request merging in attempt_merge().

For a re-queued request, the request structure is the one embedded in
the Scsi_Cmnd (which is a copy of the request taken in the
scsi_request_fn).
In attempt_merge(), q->merge_requests_fn() is called to see the requests
are allowed to merge. __scsi_merge_requests_fn() checks number of
segments, etc, but doesn't check if one of the requests is a re-queued one
(ie. no test against ->special).
This can lead to attempt_merge() releasing the embedded request
structure (which, as an extract copy, has the ->q set, so to
blkdev_release_request() it looks like a request which originated from
the block layer). This isn't too healthy.

The fix here is to add a check in __scsi_merge_requests_fn() to check
for ->special being non-NULL.

The attached patch is against 2.4.3.

Jens, Eric, anyone, comments?

Mark
diff -urN -X dontdiff linux-2.4.3/drivers/scsi/scsi_lib.c markhe-2.4.3/drivers/scsi/scsi_lib.c
--- linux-2.4.3/drivers/scsi/scsi_lib.c Sat Mar 3 02:38:39 2001
+++ markhe-2.4.3/drivers/scsi/scsi_lib.c Sat Mar 31 16:56:31 2001
@@ -377,12 +377,15 @@
nsect = bh->b_size >> 9;
blk_finished_io(nsect);
req->bh = bh->b_reqnext;
- req->nr_sectors -= nsect;
- req->sector += nsect;
bh->b_reqnext = NULL;
sectors -= nsect;
bh->b_end_io(bh, uptodate);
if ((bh = req->bh) != NULL) {
+ req->hard_sector += nsect;
+ req->hard_nr_sectors -= nsect;
+ req->sector += nsect;
+ req->nr_sectors -= nsect;
+
req->current_nr_sectors = bh->b_size >> 9;
if (req->nr_sectors < req->current_nr_sectors) {
req->nr_sectors = req->current_nr_sectors;
diff -urN -X dontdiff linux-2.4.3/drivers/scsi/scsi_merge.c markhe-2.4.3/drivers/scsi/scsi_merge.c
--- linux-2.4.3/drivers/scsi/scsi_merge.c Fri Feb 9 19:30:23 2001
+++ markhe-2.4.3/drivers/scsi/scsi_merge.c Sat Mar 31 16:38:27 2001
@@ -597,6 +597,13 @@
Scsi_Device *SDpnt;
struct Scsi_Host *SHpnt;

+ /*
+ * First check if the either of the requests are re-queued
+ * requests. Can't merge them if they are.
+ */
+ if (req->special || next->special)
+ return 0;
+
SDpnt = (Scsi_Device *) q->queuedata;
SHpnt = SDpnt->host;
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