[dm-devel] updated Chandra's patch for dm_any_congested

[Mikulas Patocka]

Hi

This is the updated patch for dm_any_congested, the only change is that
atomic_dec(&md->pending) is changed into
if (!atomic_dec_return(&md->pending)) wake_up(&md->wait);

--- so that there is not a possibility to deadlock device suspend that 
wait for pending variable to drop to zero.

You can put it into your series as a quick "stop crash" patch, until we 
find a better solution as we talked about today.

BTW. if you some times ago wondered why I'm using yield() and not waiting 
queues in performance non-critical paths, it's to avoid bugs like this :)

Mikulas
-------------- next part --------------
From: Chandra Seetharaman <sekharan at us.ibm.com>

dm_any_congested() just checks for the DMF_BLOCK_IO and has no
code to make sure that suspend waits for dm_any_congested() to
complete.  This patch adds such a check.

Without it, a race can occur with dm_table_put() attempting to
destroying the table in the wrong thread, the one running
dm_any_congested() which is meant to be quick and return
immediately.

Two examples of problems:
1. Sleeping functions called from congested code, the caller
   of which holds a spin lock.
2. An ABBA deadlock between pdflush and multipathd. The two locks
   in contention are inode lock and kernel lock.

[AGK: Is the return value always correct when the table is skipped?]

Signed-off-by: Chandra Seetharaman <sekharan at us.ibm.com>
Signed-off-by: Mikulas Patocka <mpatocka at redhat.com>
Signed-off-by: Alasdair G Kergon <agk at redhat.com>
---
 drivers/md/dm.c |   20 +++++++++++++-------
 1 file changed, 13 insertions(+), 7 deletions(-)

Index: linux-2.6.28-rc3-devel/drivers/md/dm.c
===================================================================
--- linux-2.6.28-rc3-devel.orig/drivers/md/dm.c	2008-11-05 09:19:18.000000000 +0100
+++ linux-2.6.28-rc3-devel/drivers/md/dm.c	2008-11-10 18:05:24.000000000 +0100
@@ -941,16 +941,22 @@ static void dm_unplug_all(struct request
 
 static int dm_any_congested(void *congested_data, int bdi_bits)
 {
-	int r;
+	int r = bdi_bits;
 	struct mapped_device *md = (struct mapped_device *) congested_data;
-	struct dm_table *map = dm_get_table(md);
+	struct dm_table *map;
 
-	if (!map || test_bit(DMF_BLOCK_IO, &md->flags))
-		r = bdi_bits;
-	else
-		r = dm_table_any_congested(map, bdi_bits);
+	atomic_inc(&md->pending);
+	if (test_bit(DMF_BLOCK_IO, &md->flags))
+		goto done;
 
-	dm_table_put(map);
+	map = dm_get_table(md);
+	if (map) {
+		r = dm_table_any_congested(map, bdi_bits);
+		dm_table_put(map);
+	}
+done:
+	if (!atomic_dec_return(&md->pending))
+		wake_up(&md->wait);
 	return r;
 }