Re: Re: [PATCH] Fix Null pointer Exception

[Alasdair G Kergon <agk@redhat.com>]

On Tue, Oct 07, 2008 at 10:18:40PM +0100, Alasdair G Kergon wrote:
> Meanwhile I will not be applying that patch, though I may take a patch
> to provide clean diagnostics should a situation like this occur again.
 
I'm thinking of something like this:


From: Alasdair G Kergon <agk@redhat.com>

Detect and report buggy drivers that destroy their request_queue.

Signed-off-by: Alasdair G Kergon <agk@redhat.com>
Cc: Stefan Raspl <raspl@linux.vnet.ibm.com>
Cc: Jens Axboe <jens.axboe@oracle.com>
Cc: Andrew Morton <akpm@linux-foundation.org>

---
 drivers/md/dm-table.c |   24 ++++++++++++++++++++++--
 1 files changed, 22 insertions(+), 2 deletions(-)

Index: linux-2.6.27-rc8/drivers/md/dm-table.c
===================================================================
--- linux-2.6.27-rc8.orig/drivers/md/dm-table.c	2008-10-09 14:53:35.000000000 +0100
+++ linux-2.6.27-rc8/drivers/md/dm-table.c	2008-10-09 14:56:26.000000000 +0100
@@ -478,6 +478,13 @@ void dm_set_device_limits(struct dm_targ
 {
 	struct request_queue *q = bdev_get_queue(bdev);
 	struct io_restrictions *rs = &ti->limits;
+	char b[BDEVNAME_SIZE];
+
+	if (unlikely(!q)) {
+		DMWARN("%s: Cannot set limits for nonexistent device %s",
+		       dm_device_name(ti->table->md), bdevname(bdev, b));
+		return;
+	}
 
 	/*
 	 * Combine the device limits low.
@@ -943,7 +950,14 @@ int dm_table_any_congested(struct dm_tab
 
 	list_for_each_entry(dd, devices, list) {
 		struct request_queue *q = bdev_get_queue(dd->bdev);
-		r |= bdi_congested(&q->backing_dev_info, bdi_bits);
+		char b[BDEVNAME_SIZE];
+
+		if (likely(q))
+			r |= bdi_congested(&q->backing_dev_info, bdi_bits);
+		else
+			DMWARN_LIMIT("%s: any_congested: nonexistent device %s",
+				     dm_device_name(t->md),
+				     bdevname(dd->bdev, b));
 	}
 
 	return r;
@@ -956,8 +970,14 @@ void dm_table_unplug_all(struct dm_table
 
 	list_for_each_entry(dd, devices, list) {
 		struct request_queue *q = bdev_get_queue(dd->bdev);
+		char b[BDEVNAME_SIZE];
 
-		blk_unplug(q);
+		if (likely(q))
+			blk_unplug(q);
+		else
+			DMWARN_LIMIT("%s: Cannot unplug nonexistent device %s",
+				     dm_device_name(t->md),
+				     bdevname(dd->bdev, b));
 	}
 }