dm: avoid destroying table in dm_any_congested

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.

Signed-off-by: Chandra Seetharaman <sekharan@us.ibm.com>
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>

1 files changed, 16 insertions, 8 deletions

diff --git a/drivers/md/dm.c b/drivers/md/dm.c
index dc25d8a07bc7..c99e4728ff41 100644
--- a/drivers/md/dm.c
+++ b/drivers/md/dm.c
@@ -937,16 +937,24 @@ static void dm_unplug_all(struct request_queue *q)
 
 static int dm_any_congested(void *congested_data, int bdi_bits)
 {
-        int r;
-        struct mapped_device *md = (struct mapped_device *) congested_data;
-        struct dm_table *map = dm_get_table(md);
+        int r = bdi_bits;
+        struct mapped_device *md = congested_data;
+        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)) {
+                map = dm_get_table(md);
+                if (map) {
+                        r = dm_table_any_congested(map, bdi_bits);
+                        dm_table_put(map);
+                }
+        }
+
+        if (!atomic_dec_return(&md->pending))
+                /* nudge anyone waiting on suspend queue */
+                wake_up(&md->wait);
 
-        dm_table_put(map);
         return r;
 }