MDEV-22523 index->rtr_ssn.mutex is wasting memory

As part of the SPATIAL INDEX implementation in InnoDB,
dict_index_t was expanded by a rtr_ssn_t field. There are only
3 operations for this field, all protected by rtr_ssn_t::mutex:

* btr_cur_search_to_nth_level() stores the least significant 32 bits
of the 64-bit value that is stored in the index root page.
(This would better be done when the table is opened for the
very first time.)
* rtr_get_new_ssn_id() increments the value by 1.
* rtr_get_current_ssn_id() reads the current value.

All these operations can be implemented equally safely by using
atomic memory access operations.

storage/innobase/btr/btr0cur.cc

@@ -1475,10 +1475,9 @@ btr_cur_search_to_nth_level(
 				node_seq_t      root_seq_no;
 
 				root_seq_no = page_get_ssn_id(page);
-
-				mutex_enter(&(index->rtr_ssn.mutex));
-				index->rtr_ssn.seq_no = root_seq_no + 1;
-				mutex_exit(&(index->rtr_ssn.mutex));
+				my_atomic_store32_explicit(
+					&index->rtr_ssn, root_seq_no + 1,
+					MY_MEMORY_ORDER_RELAXED);
 			}
 
 			/* Save the MBR */

storage/innobase/dict/dict0mem.cc

@@ -741,7 +741,6 @@ dict_mem_index_create(
 	dict_index_zip_pad_mutex_create_lazy(index);
 
 	if (type & DICT_SPATIAL) {
-		mutex_create(LATCH_ID_RTR_SSN_MUTEX, &index->rtr_ssn.mutex);
 		index->rtr_track = static_cast<rtr_info_track_t*>(
 					mem_heap_alloc(
 						heap,
@@ -1067,7 +1066,6 @@ dict_mem_index_free(
 			rtr_info->index = NULL;
 		}
 
-		mutex_destroy(&index->rtr_ssn.mutex);
 		mutex_destroy(&index->rtr_track->rtr_active_mutex);
 		UT_DELETE(index->rtr_track->rtr_active);
 	}

storage/innobase/handler/ha_innodb.cc

@@ -626,7 +626,6 @@ static PSI_mutex_info all_innodb_mutexes[] = {
 	PSI_KEY(rtr_active_mutex),
 	PSI_KEY(rtr_match_mutex),
 	PSI_KEY(rtr_path_mutex),
-	PSI_KEY(rtr_ssn_mutex),
 	PSI_KEY(trx_sys_mutex),
 	PSI_KEY(zip_pad_mutex)
 };

storage/innobase/include/dict0mem.h

@@ -2,7 +2,7 @@
 
 Copyright (c) 1996, 2017, Oracle and/or its affiliates. All Rights Reserved.
 Copyright (c) 2012, Facebook Inc.
-Copyright (c) 2013, 2019, MariaDB Corporation.
+Copyright (c) 2013, 2020, MariaDB Corporation.
 
 This program is free software; you can redistribute it and/or modify it under
 the terms of the GNU General Public License as published by the Free Software
@@ -964,7 +964,8 @@ struct dict_index_t{
 				/* in which slot the next sample should be
 				saved. */
 	/* @} */
-	rtr_ssn_t	rtr_ssn;/*!< Node sequence number for RTree */
+	/** R-tree split sequence number */
+	volatile int32	rtr_ssn;
 	rtr_info_track_t*
 			rtr_track;/*!< tracking all R-Tree search cursors */
 	trx_id_t	trx_id; /*!< id of the transaction that created this

storage/innobase/include/gis0rtree.ic

 /*****************************************************************************
 
 Copyright (c) 2014, Oracle and/or its affiliates. All Rights Reserved.
-Copyright (c) 2017, MariaDB Corporation.
+Copyright (c) 2017, 2020, MariaDB Corporation.
 
 This program is free software; you can redistribute it and/or modify it under
 the terms of the GNU General Public License as published by the Free Software
@@ -132,13 +132,9 @@ rtr_get_new_ssn_id(
 /*===============*/
 	dict_index_t*	index)	/*!< in/out: the index struct */
 {
-	node_seq_t	ssn;
-
-	mutex_enter(&(index->rtr_ssn.mutex));
-	ssn = ++index->rtr_ssn.seq_no;
-	mutex_exit(&(index->rtr_ssn.mutex));
-
-	return(ssn);
+  node_seq_t ssn= my_atomic_add32_explicit(&index->rtr_ssn, 1,
+                                           MY_MEMORY_ORDER_RELAXED);
+  return ssn + 1;
 }
 /*****************************************************************//**
 Get the current Split Sequence Number.
@@ -149,13 +145,7 @@ rtr_get_current_ssn_id(
 /*===================*/
 	dict_index_t*	index)	/*!< in: index struct */
 {
-	node_seq_t	ssn;
-
-	mutex_enter(&(index->rtr_ssn.mutex));
-	ssn = index->rtr_ssn.seq_no;
-	mutex_exit(&(index->rtr_ssn.mutex));
-
-	return(ssn);
+  return my_atomic_load32_explicit(&index->rtr_ssn, MY_MEMORY_ORDER_RELAXED);
 }
 
 /*********************************************************************//**

storage/innobase/include/gis0type.h

 /*****************************************************************************
 
 Copyright (c) 2014, Oracle and/or its affiliates. All Rights Reserved.
-Copyright (c) 2018, MariaDB Corporation.
+Copyright (c) 2018, 2020, MariaDB Corporation.
 
 This program is free software; you can redistribute it and/or modify it under
 the terms of the GNU General Public License as published by the Free Software
@@ -143,12 +143,6 @@ typedef struct	rtr_info_track {
 						rtr_active */
 } rtr_info_track_t;
 
-/* Node Sequence Number and mutex protects it. */
-typedef struct rtree_ssn {
-        ib_mutex_t      mutex;          /*!< mutex protect the seq num */
-        node_seq_t      seq_no;         /*!< the SSN (node sequence number) */
-} rtr_ssn_t;
-
 /* This is to record the record movement between pages. Used for corresponding
 lock movement */
 typedef struct rtr_rec_move {

storage/innobase/include/sync0sync.h

@@ -3,6 +3,7 @@
 Copyright (c) 1995, 2016, Oracle and/or its affiliates. All Rights Reserved.
 Copyright (c) 2008, Google Inc.
 Copyright (c) 2012, Facebook Inc.
+Copyright (c) 2020, MariaDB Corporation.
 
 Portions of this file contain modifications contributed and copyrighted by
 Google, Inc. Those modifications are gratefully acknowledged and are described
@@ -81,7 +82,6 @@ extern mysql_pfs_key_t	recv_writer_mutex_key;
 extern mysql_pfs_key_t	rtr_active_mutex_key;
 extern mysql_pfs_key_t	rtr_match_mutex_key;
 extern mysql_pfs_key_t	rtr_path_mutex_key;
-extern mysql_pfs_key_t	rtr_ssn_mutex_key;
 extern mysql_pfs_key_t	redo_rseg_mutex_key;
 extern mysql_pfs_key_t	noredo_rseg_mutex_key;
 extern mysql_pfs_key_t page_zip_stat_per_index_mutex_key;

storage/innobase/include/sync0types.h

 /*****************************************************************************
 
 Copyright (c) 1995, 2016, Oracle and/or its affiliates. All Rights Reserved.
-Copyright (c) 2017, 2018, MariaDB Corporation.
+Copyright (c) 2017, 2020, MariaDB Corporation.
 
 This program is free software; you can redistribute it and/or modify it under
 the terms of the GNU General Public License as published by the Free Software
@@ -327,7 +327,6 @@ enum latch_id_t {
 	LATCH_ID_REDO_RSEG,
 	LATCH_ID_NOREDO_RSEG,
 	LATCH_ID_RW_LOCK_DEBUG,
-	LATCH_ID_RTR_SSN_MUTEX,
 	LATCH_ID_RTR_ACTIVE_MUTEX,
 	LATCH_ID_RTR_MATCH_MUTEX,
 	LATCH_ID_RTR_PATH_MUTEX,

storage/innobase/sync/sync0debug.cc

 /*****************************************************************************
 
 Copyright (c) 2014, 2016, Oracle and/or its affiliates. All Rights Reserved.
-Copyright (c) 2017, 2018, MariaDB Corporation.
+Copyright (c) 2017, 2020, MariaDB Corporation.
 
 Portions of this file contain modifications contributed and copyrighted by
 Google, Inc. Those modifications are gratefully acknowledged and are described
@@ -383,8 +383,7 @@ struct LatchDebug {
 	{
 		return(latch->get_id() == LATCH_ID_RTR_ACTIVE_MUTEX
 		       || latch->get_id() == LATCH_ID_RTR_PATH_MUTEX
-		       || latch->get_id() == LATCH_ID_RTR_MATCH_MUTEX
-		       || latch->get_id() == LATCH_ID_RTR_SSN_MUTEX);
+		       || latch->get_id() == LATCH_ID_RTR_MATCH_MUTEX);
 	}
 
 private:
@@ -1375,8 +1374,6 @@ sync_latch_meta_init()
 			rw_lock_debug_mutex_key);
 #endif /* UNIV_DEBUG */
 
-	LATCH_ADD_MUTEX(RTR_SSN_MUTEX, SYNC_ANY_LATCH, rtr_ssn_mutex_key);
-
 	LATCH_ADD_MUTEX(RTR_ACTIVE_MUTEX, SYNC_ANY_LATCH,
 			rtr_active_mutex_key);
 

storage/innobase/sync/sync0sync.cc

@@ -2,6 +2,7 @@
 
 Copyright (c) 1995, 2016, Oracle and/or its affiliates. All Rights Reserved.
 Copyright (c) 2008, Google Inc.
+Copyright (c) 2020, MariaDB Corporation.
 
 Portions of this file contain modifications contributed and copyrighted by
 Google, Inc. Those modifications are gratefully acknowledged and are described
@@ -73,7 +74,6 @@ mysql_pfs_key_t	rw_lock_debug_mutex_key;
 mysql_pfs_key_t rtr_active_mutex_key;
 mysql_pfs_key_t	rtr_match_mutex_key;
 mysql_pfs_key_t	rtr_path_mutex_key;
-mysql_pfs_key_t rtr_ssn_mutex_key;
 mysql_pfs_key_t	rw_lock_list_mutex_key;
 mysql_pfs_key_t	rw_lock_mutex_key;
 mysql_pfs_key_t	srv_innodb_monitor_mutex_key;