dm cache: fix issue with background work locking

dm cache used a rw_semaphore for background_work_lock. Write locks on
rw_semaphores have strict owner semantics, but there was no guarantee
that the process that locked background_work_lock was the same process
that unlocked it. This can be easily seen using a kernel compiled with
CONFIG_DEBUG_RWSEMS. Given a dm cache device <cache>, run: 'dmsetup
suspend <cache> && dmsetup resume <cache>'. This will trigger a kernel
warning:

DEBUG_RWSEMS_WARN_ON((rwsem_owner(sem) != current) && !rwsem_test_oflags(sem, RWSEM_NONSPINNABLE))

triggered by cache_resume(). To fix this, switch from a rw_semaphore to
a spinlock and a wait queue. dm cache already has a wait queue and
associated counter, migration_wait and nr_allocated_migrations, that was
getting woken up when background work was getting completed, but wasn't
actually used by anything. This is replaced by the background_work queue
and counter.

Fixes: b29d4986d0da ("dm cache: significant rework to leverage dm-bio-prison-v2")
Cc: stable@vger.kernel.org
Signed-off-by: Benjamin Marzinski <bmarzins@redhat.com>
Reviewed-by: Matthew Sakai <msakai@redhat.com>
Reviewed-by: Ming-Hung Tsai <mtsai@redhat.com>
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>

1 files changed, 28 insertions, 27 deletions

diff --git a/drivers/md/dm-cache-target.c b/drivers/md/dm-cache-target.c
index 33dbc71b730f..d8d9c63d2a67 100644
--- a/drivers/md/dm-cache-target.c
+++ b/drivers/md/dm-cache-target.c
@@ -340,8 +340,6 @@ struct cache {
 	struct list_head invalidation_requests;
 
 	sector_t migration_threshold;
-	wait_queue_head_t migration_wait;
-	atomic_t nr_allocated_migrations;
 
 	/*
 	 * The number of in flight migrations that are performing
@@ -397,7 +395,11 @@ struct cache {
 	bool loaded_mappings:1;
 	bool loaded_discards:1;
 
-	struct rw_semaphore background_work_lock;
+	/* background work management */
+	bool background_work_allowed;
+	unsigned background_work_nr;
+	spinlock_t background_work_lock;
+	wait_queue_head_t background_work_wait;
 
 	struct batcher committer;
 	struct work_struct commit_ws;
@@ -488,19 +490,13 @@ static struct dm_cache_migration *alloc_migration(struct cache *cache)
 	memset(mg, 0, sizeof(*mg));
 
 	mg->cache = cache;
-	atomic_inc(&cache->nr_allocated_migrations);
 
 	return mg;
 }
 
 static void free_migration(struct dm_cache_migration *mg)
 {
-	struct cache *cache = mg->cache;
-
-	if (atomic_dec_and_test(&cache->nr_allocated_migrations))
-		wake_up(&cache->migration_wait);
-
-	mempool_free(mg, &cache->migration_pool);
+	mempool_free(mg, &mg->cache->migration_pool);
 }
 
 /*----------------------------------------------------------------*/
@@ -1030,34 +1026,39 @@ static void calc_discard_block_range(struct cache *cache, struct bio *bio,
 
 static void prevent_background_work(struct cache *cache)
 {
-	lockdep_off();
-	down_write(&cache->background_work_lock);
-	lockdep_on();
+	spin_lock_irq(&cache->background_work_lock);
+	cache->background_work_allowed = false;
+	wait_event_lock_irq(cache->background_work_wait,
+			    cache->background_work_nr == 0,
+			    cache->background_work_lock);
+	spin_unlock_irq(&cache->background_work_lock);
 }
 
 static void allow_background_work(struct cache *cache)
 {
-	lockdep_off();
-	up_write(&cache->background_work_lock);
-	lockdep_on();
+	spin_lock_irq(&cache->background_work_lock);
+	cache->background_work_allowed = true;
+	spin_unlock_irq(&cache->background_work_lock);
 }
 
 static bool background_work_begin(struct cache *cache)
 {
 	bool r;
 
-	lockdep_off();
-	r = down_read_trylock(&cache->background_work_lock);
-	lockdep_on();
-
+	spin_lock_irq(&cache->background_work_lock);
+	r = cache->background_work_allowed;
+	if (r)
+		cache->background_work_nr++;
+	spin_unlock_irq(&cache->background_work_lock);
 	return r;
 }
 
 static void background_work_end(struct cache *cache)
 {
-	lockdep_off();
-	up_read(&cache->background_work_lock);
-	lockdep_on();
+	spin_lock_irq(&cache->background_work_lock);
+	if (--cache->background_work_nr == 0)
+		wake_up(&cache->background_work_wait);
+	spin_unlock_irq(&cache->background_work_lock);
 }
 
 /*----------------------------------------------------------------*/
@@ -2507,9 +2508,7 @@ static int cache_create(struct cache_args *ca, struct cache **result)
 
 	spin_lock_init(&cache->lock);
 	bio_list_init(&cache->deferred_bios);
-	atomic_set(&cache->nr_allocated_migrations, 0);
 	atomic_set(&cache->nr_io_migrations, 0);
-	init_waitqueue_head(&cache->migration_wait);
 
 	r = -ENOMEM;
 	atomic_set(&cache->nr_dirty, 0);
@@ -2592,8 +2591,10 @@ static int cache_create(struct cache_args *ca, struct cache **result)
 		     issue_op, cache, cache->wq);
 	dm_iot_init(&cache->tracker);
 
-	init_rwsem(&cache->background_work_lock);
-	prevent_background_work(cache);
+	init_waitqueue_head(&cache->background_work_wait);
+	spin_lock_init(&cache->background_work_lock);
+	cache->background_work_allowed = false;
+	cache->background_work_nr = 0;
 
 	*result = cache;
 	return 0;