On Mon, Mar 23, 2009 at 05:00:36PM +1100, Neil Brown wrote:
> The code in current mainline is exactly the same so if we are correct
> in our assessment, then the bug is still present.
> Our current patch is below. It is a big ugly, and a better fix might
> be a more thorough rewrite of the code. However I offer it incase it
> is useful.
Indeed this area of code is in need of a clean up.
Here's an alternative version trying additionally to avoid the theoretical risk
from overwriting the field with the same value.
Alasdair
From: Alasdair G Kergon <agk@xxxxxxxxxx>
upgrade_mode() sets bdev to NULL temporarily, and does not have any
locking to exclude anything from seeing that NULL.
In dm_table_any_congested() bdev_get_queue() can dereference that NULL and
cause a reported oops.
Fix this by not changing that field during the mode upgrade.
Cc: Neil Brown <neilb@xxxxxxx>
Signed-off-by: Alasdair G Kergon <agk@xxxxxxxxxx>
---
drivers/md/dm-table.c | 26 ++++++++++++++------------
1 file changed, 14 insertions(+), 12 deletions(-)
Index: linux-2.6.29/drivers/md/dm-table.c
===================================================================
--- linux-2.6.29.orig/drivers/md/dm-table.c
+++ linux-2.6.29/drivers/md/dm-table.c
@@ -399,28 +399,30 @@ static int check_device_area(struct dm_d
}
/*
- * This upgrades the mode on an already open dm_dev. Being
+ * This upgrades the mode on an already open dm_dev, being
* careful to leave things as they were if we fail to reopen the
- * device.
+ * device and not to touch the existing bdev field in case
+ * it is accessed concurrently inside dm_table_any_congested().
*/
static int upgrade_mode(struct dm_dev_internal *dd, fmode_t new_mode,
struct mapped_device *md)
{
int r;
- struct dm_dev_internal dd_copy;
- dev_t dev = dd->dm_dev.bdev->bd_dev;
+ struct dm_dev_internal dd_new, dd_old;
- dd_copy = *dd;
+ dd_new = dd_old = *dd;
+
+ dd_new.dm_dev.mode |= new_mode;
+ dd_new.dm_dev.bdev = NULL;
+
+ r = open_dev(&dd_new, dd->dm_dev.bdev->bd_dev, md);
+ if (r)
+ return r;
dd->dm_dev.mode |= new_mode;
- dd->dm_dev.bdev = NULL;
- r = open_dev(dd, dev, md);
- if (!r)
- close_dev(&dd_copy, md);
- else
- *dd = dd_copy;
+ close_dev(&dd_old, md);
- return r;
+ return 0;
}
/*
--
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