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dm cache: fix a lock-inversion

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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
This commit is contained in:
Joe Thornber
2014-04-03 16:16:44 +01:00
committed by Mike Snitzer
parent 67324ea188
commit 0596661f0a
3 changed files with 21 additions and 53 deletions
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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,