MDEV-23651 InnoDB: Failing assertion: !space->referenced()

commit de942c9f (MDEV-15983)
introduced a race condition that we inadequately fixed in
commit 93b69825 (MDEV-16169).

Because fil_space_t::release() or fil_space_t::acquire() are
not protected by fil_system.mutex like their predecessors,
it is possible that stop_new_ops was set between the time
a thread checked fil_space_t::is_stopping() and invoked
fil_space_t::acquire().

In an execution trace, this happened in fil_system_t::keyrotate_next(),
causing an assertion failure in fil_delete_tablespace()
in the other thread that seeked to stop new operations.

We fix this bug by merging the flag fil_space_t::stop_new_ops
and the reference count fil_space_t::n_pending_ops into a
single word that is only being accessed by atomic memory operations.

fil_space_t::set_stopping(): Accessor for changing the state of
the former stop_new_ops flag.

fil_space_t::acquire(): Return whether the acquisition succeeded.
It would fail between set_stopping(true) and set_stopping(false).

storage/innobase/fil/fil0crypt.cc

@@ -1418,12 +1418,15 @@ inline fil_space_t *fil_system_t::keyrotate_next(fil_space_t *space,
     }
   }
 
-  if (it == end)
-    return NULL;
-
+  while (it != end)
+  {
     space= &*it;
-  space->acquire();
+    if (space->acquire())
       return space;
+    while (++it != end && (!UT_LIST_GET_LEN(it->chain) || it->is_stopping()));
+  }
+
+  return NULL;
 }
 
 /** Return the next tablespace.
@@ -1445,12 +1448,14 @@ static fil_space_t *fil_space_next(fil_space_t *space, bool recheck,
     space= UT_LIST_GET_FIRST(fil_system.space_list);
     /* We can trust that space is not NULL because at least the
     system tablespace is always present and loaded first. */
-    space->acquire();
+    if (!space->acquire())
+      goto next;
   }
   else
   {
     /* Move on to the next fil_space_t */
     space->release();
+next:
     space= UT_LIST_GET_NEXT(space_list, space);
 
     /* Skip abnormal tablespaces or those that are being created by
@@ -1460,8 +1465,8 @@ static fil_space_t *fil_space_next(fil_space_t *space, bool recheck,
             space->is_stopping() || space->purpose != FIL_TYPE_TABLESPACE))
       space= UT_LIST_GET_NEXT(space_list, space);
 
-    if (space)
-      space->acquire();
+    if (space && !space->acquire())
+      goto next;
   }
 
   mutex_exit(&fil_system.mutex);
@@ -2330,31 +2335,27 @@ static void fil_crypt_rotation_list_fill()
 	     space = UT_LIST_GET_NEXT(space_list, space)) {
 		if (space->purpose != FIL_TYPE_TABLESPACE
 		    || space->is_in_rotation_list
-		    || space->is_stopping()
-		    || UT_LIST_GET_LEN(space->chain) == 0) {
+		    || UT_LIST_GET_LEN(space->chain) == 0
+		    || !space->acquire()) {
 			continue;
 		}
 
 		/* Ensure that crypt_data has been initialized. */
 		if (!space->size) {
-			/* Protect the tablespace while we may
-			release fil_system.mutex. */
-			ut_d(space->acquire());
 			ut_d(const fil_space_t* s=)
 			        fil_system.read_page0(space->id);
 			ut_ad(!s || s == space);
-			ut_d(space->release());
 			if (!space->size) {
 				/* Page 0 was not loaded.
 				Skip this tablespace. */
-				continue;
+				goto next;
 			}
 		}
 
 		/* Skip ENCRYPTION!=DEFAULT tablespaces. */
 		if (space->crypt_data
 		    && !space->crypt_data->is_default_encryption()) {
-			continue;
+			goto next;
 		}
 
 		if (srv_encrypt_tables) {
@@ -2362,19 +2363,21 @@ static void fil_crypt_rotation_list_fill()
 			innodb_encrypt_tables!=OFF */
 			if (space->crypt_data
 			    && space->crypt_data->min_key_version) {
-				continue;
+				goto next;
 			}
 		} else {
 			/* Skip unencrypted tablespaces if
 			innodb_encrypt_tables=OFF */
 			if (!space->crypt_data
 			    || !space->crypt_data->min_key_version) {
-				continue;
+				goto next;
 			}
 		}
 
 		fil_system.rotation_list.push_back(*space);
 		space->is_in_rotation_list = true;
+next:
+		space->release();
 	}
 }
 

storage/innobase/fil/fil0fil.cc

@@ -763,8 +763,7 @@ fil_try_to_close_file_in_LRU(
 static void fil_flush_low(fil_space_t* space, bool metadata = false)
 {
 	ut_ad(mutex_own(&fil_system.mutex));
-	ut_ad(space);
-	ut_ad(!space->stop_new_ops);
+	ut_ad(!space->is_stopping());
 
 	if (fil_buffering_disabled(space)) {
 
@@ -1934,10 +1933,8 @@ fil_space_acquire_low(ulint id, bool silent)
 			ib::warn() << "Trying to access missing"
 				" tablespace " << id;
 		}
-	} else if (space->is_stopping()) {
+	} else if (!space->acquire()) {
 		space = NULL;
-	} else {
-		space->acquire();
 	}
 
 	mutex_exit(&fil_system.mutex);
@@ -2254,11 +2251,11 @@ fil_check_pending_ops(const fil_space_t* space, ulint count)
 {
 	ut_ad(mutex_own(&fil_system.mutex));
 
-	if (space == NULL) {
+	if (!space) {
 		return 0;
 	}
 
-	if (ulint n_pending_ops = my_atomic_loadlint(&space->n_pending_ops)) {
+	if (uint32_t n_pending_ops = space->referenced()) {
 
 		/* Give a warning every 10 second, starting after 1 second */
 		if ((count % 500) == 50) {
@@ -2347,14 +2344,13 @@ fil_check_pending_operations(
 	fil_space_t* sp = fil_space_get_by_id(id);
 
 	if (sp) {
-		sp->stop_new_ops = true;
-		if (sp->crypt_data) {
-			sp->acquire();
+		if (sp->crypt_data && sp->acquire()) {
 			mutex_exit(&fil_system.mutex);
 			fil_space_crypt_close_tablespace(sp);
 			mutex_enter(&fil_system.mutex);
 			sp->release();
 		}
+		sp->set_stopping(true);
 	}
 
 	/* Check for pending operations. */
@@ -2875,7 +2871,7 @@ fil_rename_tablespace(
 	multiple datafiles per tablespace. */
 	ut_a(UT_LIST_GET_LEN(space->chain) == 1);
 	node = UT_LIST_GET_FIRST(space->chain);
-	space->n_pending_ops++;
+	ut_a(space->acquire());
 
 	mutex_exit(&fil_system.mutex);
 
@@ -2897,8 +2893,7 @@ fil_rename_tablespace(
 	/* log_sys.mutex is above fil_system.mutex in the latching order */
 	ut_ad(log_mutex_own());
 	mutex_enter(&fil_system.mutex);
-	ut_ad(space->n_pending_ops);
-	space->n_pending_ops--;
+	space->release();
 	ut_ad(space->name == old_space_name);
 	ut_ad(node->name == old_file_name);
 	bool success;
@@ -4204,7 +4199,7 @@ fil_io(
 	if (space == NULL
 	    || (req_type.is_read()
 		&& !sync
-		&& space->stop_new_ops
+		&& space->is_stopping()
 		&& !space->is_being_truncated)) {
 
 		mutex_exit(&fil_system.mutex);
@@ -4844,7 +4839,7 @@ fil_space_validate_for_mtr_commit(
 	fil_space_acquire() before mtr_start() and
 	fil_space_t::release() after mtr_commit(). This is why
 	n_pending_ops should not be zero if stop_new_ops is set. */
-	ut_ad(!space->stop_new_ops
+	ut_ad(!space->is_stopping()
 	      || space->is_being_truncated /* fil_truncate_prepare() */
 	      || space->referenced());
 }
@@ -5070,7 +5065,7 @@ truncate_t::truncate(
 		err = DB_ERROR;
 	}
 
-	space->stop_new_ops = false;
+	space->set_stopping(false);
 
 	/* If we opened the file in this function, close it. */
 	if (!already_open) {
@@ -5171,26 +5166,6 @@ fil_space_get_block_size(const fil_space_t* space, unsigned offset)
 	return block_size;
 }
 
-/*******************************************************************//**
-Returns the table space by a given id, NULL if not found. */
-fil_space_t*
-fil_space_found_by_id(
-/*==================*/
-	ulint	id)	/*!< in: space id */
-{
-	fil_space_t* space = NULL;
-	mutex_enter(&fil_system.mutex);
-	space = fil_space_get_by_id(id);
-
-	/* Not found if space is being deleted */
-	if (space && space->stop_new_ops) {
-		space = NULL;
-	}
-
-	mutex_exit(&fil_system.mutex);
-	return space;
-}
-
 /**
 Get should we punch hole to tablespace.
 @param[in]	node		File node

storage/innobase/handler/i_s.cc

@@ -8579,8 +8579,7 @@ i_s_tablespaces_encryption_fill_table(
 	for (fil_space_t* space = UT_LIST_GET_FIRST(fil_system.space_list);
 	     space; space = UT_LIST_GET_NEXT(space_list, space)) {
 		if (space->purpose == FIL_TYPE_TABLESPACE
-		    && !space->is_stopping()) {
-			space->acquire();
+		    && space->acquire()) {
 			mutex_exit(&fil_system.mutex);
 			if (int err = i_s_dict_fill_tablespaces_encryption(
 				    thd, space, tables->table)) {
@@ -8842,8 +8841,7 @@ i_s_tablespaces_scrubbing_fill_table(
 	for (fil_space_t* space = UT_LIST_GET_FIRST(fil_system.space_list);
 	     space; space = UT_LIST_GET_NEXT(space_list, space)) {
 		if (space->purpose == FIL_TYPE_TABLESPACE
-		    && !space->is_stopping()) {
-			space->acquire();
+		    && space->acquire()) {
 			mutex_exit(&fil_system.mutex);
 			if (int err = i_s_dict_fill_tablespaces_scrubbing(
 				    thd, space, tables->table)) {

storage/innobase/include/fil0fil.h

@@ -93,18 +93,6 @@ struct fil_space_t : ilist_node<unflushed_spaces_tag_t>,
 	/** Log sequence number of the latest MLOG_INDEX_LOAD record
 	that was found while parsing the redo log */
 	lsn_t		enable_lsn;
-	bool		stop_new_ops;
-				/*!< we set this true when we start
-				deleting a single-table tablespace.
-				When this is set following new ops
-				are not allowed:
-				* read IO request
-				* ibuf merge
-				* file flush
-				Note that we can still possibly have
-				new write operations because we don't
-				check this flag when doing flush
-				batches. */
 	/** whether undo tablespace truncation is in progress */
 	bool		is_being_truncated;
 #ifdef UNIV_DEBUG
@@ -142,14 +130,24 @@ struct fil_space_t : ilist_node<unflushed_spaces_tag_t>,
 	ulint		n_pending_flushes; /*!< this is positive when flushing
 				the tablespace to disk; dropping of the
 				tablespace is forbidden if this is positive */
-	/** Number of pending buffer pool operations accessing the tablespace
-	without holding a table lock or dict_operation_lock S-latch
-	that would prevent the table (and tablespace) from being
+private:
+  /** Number of pending buffer pool operations accessing the
+  tablespace without holding a table lock or dict_operation_lock
+  S-latch that would prevent the table (and tablespace) from being
   dropped. An example is change buffer merge.
-	The tablespace cannot be dropped while this is nonzero,
-	or while fil_node_t::n_pending is nonzero.
-	Protected by fil_system.mutex and my_atomic_loadlint() and friends. */
-	ulint		n_pending_ops;
+
+  The tablespace cannot be dropped while this is nonzero, or while
+  fil_node_t::n_pending is nonzero.
+
+  The most significant bit contains the STOP_NEW_OPS flag.
+
+  Protected by my_atomic. */
+  uint32_t n_pending_ops;
+
+  /** Flag in n_pending_ops that indicates that the tablespace is being
+  deleted, and no further operations should be performed */
+  static const uint32_t STOP_NEW_OPS= 1U << 31;
+public:
 	/** Number of pending block read or write operations
 	(when a write is imminent or a read has recently completed).
 	The tablespace object cannot be freed while this is nonzero,
@@ -183,9 +181,6 @@ struct fil_space_t : ilist_node<unflushed_spaces_tag_t>,
 
 	ulint		magic_n;/*!< FIL_SPACE_MAGIC_N */
 
-	/** @return whether the tablespace is about to be dropped */
-	bool is_stopping() const { return stop_new_ops;	}
-
   /** Clamp a page number for batched I/O, such as read-ahead.
   @param offset   page number limit
   @return offset clamped to the tablespace size */
@@ -267,22 +262,61 @@ struct fil_space_t : ilist_node<unflushed_spaces_tag_t>,
 	/** Close each file. Only invoked on fil_system.temp_space. */
 	void close();
 
-	/** Acquire a tablespace reference. */
-	void acquire() { my_atomic_addlint(&n_pending_ops, 1); }
+  /** @return whether the tablespace is about to be dropped or is referenced */
+  uint32_t is_stopping_or_referenced()
+  {
+    return my_atomic_load32(&n_pending_ops);
+  }
+
+  /** @return whether the tablespace is about to be dropped or is referenced */
+  uint32_t is_stopping_or_referenced() const
+  {
+    return const_cast<fil_space_t*>(this)->is_stopping_or_referenced();
+  }
+
+  /** @return whether the tablespace is about to be dropped */
+  bool is_stopping() const
+  {
+    return is_stopping_or_referenced() & STOP_NEW_OPS;
+  }
+
+  /** @return number of references being held */
+  uint32_t referenced() const
+  {
+    return is_stopping_or_referenced() & ~STOP_NEW_OPS;
+  }
+
+  /** Note that operations on the tablespace must stop or can resume */
+  void set_stopping(bool stopping)
+  {
+    /* Note: starting with 10.4 this should be std::atomic::fetch_xor() */
+    uint32_t n= stopping ? 0 : STOP_NEW_OPS;
+    while (!my_atomic_cas32_strong_explicit(&n_pending_ops, &n,
+                                            n ^ STOP_NEW_OPS,
+                                            MY_MEMORY_ORDER_ACQUIRE,
+                                            MY_MEMORY_ORDER_RELAXED))
+      ut_ad(!(n & STOP_NEW_OPS) == stopping);
+  }
+
+  MY_ATTRIBUTE((warn_unused_result))
+  /** @return whether a tablespace reference was successfully acquired */
+  bool acquire()
+  {
+    uint32_t n= 0;
+    while (!my_atomic_cas32_strong_explicit(&n_pending_ops, &n, n + 1,
+                                            MY_MEMORY_ORDER_ACQUIRE,
+                                            MY_MEMORY_ORDER_RELAXED))
+      if (UNIV_UNLIKELY(n & STOP_NEW_OPS))
+        return false;
+    return true;
+  }
   /** Release a tablespace reference.
   @return whether this was the last reference */
   bool release()
   {
-		ulint n = my_atomic_addlint(&n_pending_ops, ulint(-1));
-		ut_ad(n);
-		return n == 1;
-	}
-	/** @return whether references are being held */
-	bool referenced() { return my_atomic_loadlint(&n_pending_ops); }
-	/** @return whether references are being held */
-	bool referenced() const
-	{
-		return const_cast<fil_space_t*>(this)->referenced();
+    uint32_t n= my_atomic_add32(&n_pending_ops, uint32_t(-1));
+    ut_ad(n & ~STOP_NEW_OPS);
+    return (n & ~STOP_NEW_OPS) == 1;
   }
 
   /** Acquire a tablespace reference for I/O. */
@@ -290,8 +324,8 @@ struct fil_space_t : ilist_node<unflushed_spaces_tag_t>,
   /** Release a tablespace reference for I/O. */
   void release_for_io()
   {
-		ut_ad(pending_io());
-		my_atomic_addlint(&n_pending_ios, ulint(-1));
+    ut_d(ulint n=) my_atomic_addlint(&n_pending_ios, ulint(-1));
+    ut_ad(n);
   }
   /** @return whether I/O is pending */
   bool pending_io() { return my_atomic_loadlint(&n_pending_ios); }

storage/innobase/trx/trx0purge.cc

@@ -1030,13 +1030,13 @@ trx_purge_initiate_truncate(
 	/* This is only executed by the srv_purge_coordinator_thread. */
 	export_vars.innodb_undo_truncations++;
 
-	/* TODO: PUNCH_HOLE the garbage (with write-ahead logging) */
+	/* In MDEV-8319 (10.5) we will PUNCH_HOLE the garbage
+	(with write-ahead logging). */
 
 	mutex_enter(&fil_system.mutex);
-	ut_ad(space->stop_new_ops);
 	ut_ad(space->is_being_truncated);
-	space->stop_new_ops = false;
 	space->is_being_truncated = false;
+	space->set_stopping(false);
 	mutex_exit(&fil_system.mutex);
 
 	if (purge_sys.rseg != NULL