MDEV-15855: Use atomics for dict_table_t::n_ref_count

Make dict_table_t::n_ref_count private, and protect it with
a combination of dict_sys->mutex and atomics. We want to be
able to invoke dict_table_t::release() without holding
dict_sys->mutex.

storage/innobase/btr/btr0sea.cc

@@ -1337,7 +1337,7 @@ void btr_search_drop_page_hash_when_freed(const page_id_t& page_id)
 			/* In all our callers, the table handle should
 			be open, or we should be in the process of
 			dropping the table (preventing eviction). */
-			ut_ad(index->table->n_ref_count > 0
+			ut_ad(index->table->get_ref_count() > 0
 			      || mutex_own(&dict_sys->mutex));
 			btr_search_drop_page_hash_index(block);
 		}

storage/innobase/dict/dict0dict.cc

@@ -423,7 +423,7 @@ dict_table_try_drop_aborted(
 	dict_table_t*	table,		/*!< in: table, or NULL if it
 					needs to be looked up again */
 	table_id_t	table_id,	/*!< in: table identifier */
-	ulint		ref_count)	/*!< in: expected table->n_ref_count */
+	int32		ref_count)	/*!< in: expected table->n_ref_count */
 {
 	trx_t*		trx;
 
@@ -1353,7 +1353,7 @@ static
 ibool
 dict_table_can_be_evicted(
 /*======================*/
-	const dict_table_t*	table)		/*!< in: table to test */
+	dict_table_t*	table)		/*!< in: table to test */
 {
 	ut_ad(mutex_own(&dict_sys->mutex));
 	ut_ad(rw_lock_own(dict_operation_lock, RW_LOCK_X));

storage/innobase/handler/ha_innodb.cc

@@ -3058,7 +3058,7 @@ ha_innobase::update_thd(
 		   m_user_thd, thd));
 
 	/* The table should have been opened in ha_innobase::open(). */
-	DBUG_ASSERT(m_prebuilt->table->n_ref_count > 0);
+	DBUG_ASSERT(m_prebuilt->table->get_ref_count() > 0);
 
 	trx_t*	trx = check_trx_exists(thd);
 
@@ -14186,7 +14186,7 @@ ha_innobase::info_low(
 	m_prebuilt->trx->op_info = "returning various info to MariaDB";
 
 	ib_table = m_prebuilt->table;
-	DBUG_ASSERT(ib_table->n_ref_count > 0);
+	DBUG_ASSERT(ib_table->get_ref_count() > 0);
 
 	if (flag & HA_STATUS_TIME) {
 		if (is_analyze || innobase_stats_on_metadata) {

storage/innobase/include/dict0dict.ic

@@ -1498,23 +1498,13 @@ dict_table_is_file_per_table(
 	return !is_system_tablespace(table->space);
 }
 
-/** Get reference count.
-@return current value of n_ref_count */
-inline
-ulint
-dict_table_t::get_ref_count() const
-{
-	ut_ad(mutex_own(&dict_sys->mutex));
-	return(n_ref_count);
-}
-
 /** Acquire the table handle. */
 inline
 void
 dict_table_t::acquire()
 {
 	ut_ad(mutex_own(&dict_sys->mutex));
-	++n_ref_count;
+	my_atomic_add32_explicit(&n_ref_count, 1, MY_MEMORY_ORDER_RELAXED);
 }
 
 /** Release the table handle.
@@ -1523,9 +1513,10 @@ inline
 bool
 dict_table_t::release()
 {
-	ut_ad(mutex_own(&dict_sys->mutex));
-	ut_ad(n_ref_count > 0);
-	return !--n_ref_count;
+	int32 n = my_atomic_add32_explicit(
+			&n_ref_count, -1, MY_MEMORY_ORDER_RELAXED);
+	ut_ad(n > 0);
+	return n == 1;
 }
 
 /** Encode the number of columns and number of virtual columns in a

storage/innobase/include/dict0mem.h

@@ -1357,7 +1357,11 @@ struct dict_table_t {
 
 	/** Get reference count.
 	@return current value of n_ref_count */
-	inline ulint get_ref_count() const;
+	inline int32 get_ref_count()
+	{
+		return my_atomic_load32_explicit(&n_ref_count,
+						 MY_MEMORY_ORDER_RELAXED);
+	}
 
 	/** Acquire the table handle. */
 	inline void acquire();
@@ -1737,13 +1741,11 @@ struct dict_table_t {
 	It is protected by lock_sys->mutex. */
 	ulint					n_rec_locks;
 
-#ifndef UNIV_DEBUG
 private:
-#endif
 	/** Count of how many handles are opened to this table. Dropping of the
 	table is NOT allowed until this count gets to zero. MySQL does NOT
 	itself check the number of open handles at DROP. */
-	ulint					n_ref_count;
+	int32					n_ref_count;
 
 public:
 	/** List of locks on the table. Protected by lock_sys->mutex. */

storage/innobase/lock/lock0lock.cc

@@ -1493,7 +1493,7 @@ lock_rec_create_low(
 	lock_rec_bitmap_reset(lock);
 	lock_rec_set_nth_bit(lock, heap_no);
 	index->table->n_rec_locks++;
-	ut_ad(index->table->n_ref_count > 0 || !index->table->can_be_evicted);
+	ut_ad(index->table->get_ref_count() > 0 || !index->table->can_be_evicted);
 
 #ifdef WITH_WSREP
 	if (c_lock && wsrep_on_trx(trx)
@@ -3664,7 +3664,7 @@ lock_table_create(
 
 	lock->un_member.tab_lock.table = table;
 
-	ut_ad(table->n_ref_count > 0 || !table->can_be_evicted);
+	ut_ad(table->get_ref_count() > 0 || !table->can_be_evicted);
 
 	UT_LIST_ADD_LAST(trx->lock.trx_locks, lock);