dm cache: fix a lock-inversion

When suspending a cache the policy is walked and the individual policy
hints written to the metadata via sync_metadata().  This led to this
lock order:

      policy->lock
        cache_metadata->root_lock

When loading the cache target the policy is populated while the metadata
lock is held:

      cache_metadata->root_lock
         policy->lock

Fix this potential lock-inversion (ABBA) deadlock in sync_metadata() by
ensuring the cache_metadata root_lock is held whilst all the hints are
written, rather than being repeatedly locked while policy->lock is held
(as was the case with each callout that policy_walk_mappings() made to
the old save_hint() method).

Found by turning on the CONFIG_PROVE_LOCKING ("Lock debugging: prove
locking correctness") build option.  However, it is not clear how the
LOCKDEP reported paths can lead to a deadlock since the two paths,
suspending a target and loading a target, never occur at the same time.
But that doesn't mean the same lock-inversion couldn't have occurred
elsewhere.

Reported-by: Marian Csontos <mcsontos@redhat.com>
Signed-off-by: Joe Thornber <ejt@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Cc: stable@vger.kernel.org

1 files changed, 2 insertions, 26 deletions

diff --git a/drivers/md/dm-cache-target.c b/drivers/md/dm-cache-target.c
index 8534679918f4..1bf4a71919ec 100644
--- a/drivers/md/dm-cache-target.c
+++ b/drivers/md/dm-cache-target.c
@@ -2582,30 +2582,6 @@ static int write_discard_bitset(struct cache *cache)
         return 0;
 }
 
-static int save_hint(void *context, dm_cblock_t cblock, dm_oblock_t oblock,
-                     uint32_t hint)
-{
-        struct cache *cache = context;
-        return dm_cache_save_hint(cache->cmd, cblock, hint);
-}
-
-static int write_hints(struct cache *cache)
-{
-        int r;
-
-        r = dm_cache_begin_hints(cache->cmd, cache->policy);
-        if (r) {
-                DMERR("dm_cache_begin_hints failed");
-                return r;
-        }
-
-        r = policy_walk_mappings(cache->policy, save_hint, cache);
-        if (r)
-                DMERR("policy_walk_mappings failed");
-
-        return r;
-}
-
 /*
  * returns true on success
  */
@@ -2623,7 +2599,7 @@ static bool sync_metadata(struct cache *cache)
 
         save_stats(cache);
 
-        r3 = write_hints(cache);
+        r3 = dm_cache_write_hints(cache->cmd, cache->policy);
         if (r3)
                 DMERR("could not write hints");
 
@@ -3094,7 +3070,7 @@ static void cache_io_hints(struct dm_target *ti, struct queue_limits *limits)
 
 static struct target_type cache_target = {
         .name = "cache",
-        .version = {1, 3, 0},
+        .version = {1, 4, 0},
         .module = THIS_MODULE,
         .ctr = cache_ctr,
         .dtr = cache_dtr,