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dm cache: fix a lock-inversion

Browse source 
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 <[email protected]>
Signed-off-by: Joe Thornber <[email protected]>
Signed-off-by: Mike Snitzer <[email protected]>
Cc: [email protected]
This commit is contained in:
Joe Thornber 2014-04-03 16:16:44 +01:00 • committed by Mike Snitzer
commit 0596661f0a
3 changed files with 20 additions and 52 deletions
Download patch file Download diff file Expand all files Collapse all files

37
drivers/md/dm-cache-metadata.c
View file

@ -1245,22 +1245,12 @@ static int begin_hints(struct dm_cache_metadata *cmd, struct dm_cache_policy *po
return 0;
}
int dm_cache_begin_hints(struct dm_cache_metadata *cmd, struct dm_cache_policy *policy)
static int save_hint(void *context, dm_cblock_t cblock, dm_oblock_t oblock, uint32_t hint)
{
int r;
down_write(&cmd->root_lock);
r = begin_hints(cmd, policy);
up_write(&cmd->root_lock);
return r;
}
static int save_hint(struct dm_cache_metadata *cmd, dm_cblock_t cblock,
uint32_t hint)
{
int r;
struct dm_cache_metadata *cmd = context;
__le32 value = cpu_to_le32(hint);
int r;
__dm_bless_for_disk(&value);
r = dm_array_set_value(&cmd->hint_info, cmd->hint_root,
@ -1270,16 +1260,25 @@ static int save_hint(struct dm_cache_metadata *cmd, dm_cblock_t cblock,
return r;
}
int dm_cache_save_hint(struct dm_cache_metadata *cmd, dm_cblock_t cblock,
uint32_t hint)
static int write_hints(struct dm_cache_metadata *cmd, struct dm_cache_policy *policy)
{
int r;
if (!hints_array_initialized(cmd))
return 0;
r = begin_hints(cmd, policy);
if (r) {
DMERR("begin_hints failed");
return r;
}
return policy_walk_mappings(policy, save_hint, cmd);
}
int dm_cache_write_hints(struct dm_cache_metadata *cmd, struct dm_cache_policy *policy)
{
int r;
down_write(&cmd->root_lock);
r = save_hint(cmd, cblock, hint);
r = write_hints(cmd, policy);
up_write(&cmd->root_lock);
return r;

9
drivers/md/dm-cache-metadata.h
View file

@ -128,14 +128,7 @@ void dm_cache_dump(struct dm_cache_metadata *cmd);
* rather than querying the policy for each cblock, we let it walk its data
* structures and fill in the hints in whatever order it wishes.
*/
int dm_cache_begin_hints(struct dm_cache_metadata *cmd, struct dm_cache_policy *p);
/*
* requests hints for every cblock and stores in the metadata device.
*/
int dm_cache_save_hint(struct dm_cache_metadata *cmd,
dm_cblock_t cblock, uint32_t hint);
int dm_cache_write_hints(struct dm_cache_metadata *cmd, struct dm_cache_policy *p);
/*
* Query method. Are all the blocks in the cache clean?

28
drivers/md/dm-cache-target.c
View file

@ -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,