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Commit 0596661f authored by Joe Thornber's avatar Joe Thornber Committed by Mike Snitzer
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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: default avatarMarian Csontos <mcsontos@redhat.com>
Signed-off-by: default avatarJoe Thornber <ejt@redhat.com>
Signed-off-by: default avatarMike Snitzer <snitzer@redhat.com>
Cc: stable@vger.kernel.org
parent 67324ea1

drivers/md/dm-cache-metadata.c

+17 −18
Original line number Diff line number Diff line
@@ -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)

{

	struct dm_cache_metadata *cmd = context;

	__le32 value = cpu_to_le32(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;

	__le32 value = cpu_to_le32(hint);

	__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;

drivers/md/dm-cache-metadata.h

+1 −8
Original line number Diff line number Diff line
@@ -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?

drivers/md/dm-cache-target.c

+2 −26
Original line number Diff line number Diff line
@@ -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,