Minor clean-up of purge code

purge_sys_t::n_submitted: Document that it is only accessed by
srv_purge_coordinator_thread.

purge_sys_t::n_completed: Exclusively use my_atomic access.

srv_task_execute(): Simplify the code.

srv_purge_coordinator_thread(): Test the cheaper condition first.

trx_purge(): Atomically access purge_sys.n_completed.
Remove some code duplication.

trx_purge_wait_for_workers_to_complete(): Atomically access
purge_sys.n_completed. Remove an unnecessary local variable.

trx_purge_stop(): Remove a redundant assignment.

storage/innobase/include/trx0purge.h

@@ -424,9 +424,12 @@ class purge_sys_t
 	MY_ALIGNED(CACHE_LINE_SIZE)
 	ReadView	view;		/*!< The purge will not remove undo logs
 					which are >= this view (purge view) */
-	ulint	n_submitted;	/*!< Count of total tasks submitted
-					to the task queue */
-	ulint	n_completed;	/*!< Count of total tasks completed */
+	/** Total number of tasks submitted by srv_purge_coordinator_thread.
+	Not accessed by other threads. */
+	ulint	n_submitted;
+	/** Number of completed tasks. Accessed by srv_purge_coordinator
+	and srv_worker_thread by my_atomic. */
+	ulint	n_completed;
 
 	/** Iterator to the undo log records of committed transactions */
 	struct iterator

storage/innobase/srv/srv0srv.cc

@@ -2474,38 +2474,25 @@ srv_purge_should_exit(ulint n_purged)
 /*********************************************************************//**
 Fetch and execute a task from the work queue.
 @return true if a task was executed */
-static
-bool
-srv_task_execute(void)
-/*==================*/
+static bool srv_task_execute()
 {
-	que_thr_t*	thr = NULL;
-
 	ut_ad(!srv_read_only_mode);
-	ut_a(srv_force_recovery < SRV_FORCE_NO_BACKGROUND);
+	ut_ad(srv_force_recovery < SRV_FORCE_NO_BACKGROUND);
 
 	mutex_enter(&srv_sys.tasks_mutex);
 
-	if (UT_LIST_GET_LEN(srv_sys.tasks) > 0) {
-
-		thr = UT_LIST_GET_FIRST(srv_sys.tasks);
-
+	if (que_thr_t* thr = UT_LIST_GET_FIRST(srv_sys.tasks)) {
 		ut_a(que_node_get_type(thr->child) == QUE_NODE_PURGE);
-
 		UT_LIST_REMOVE(srv_sys.tasks, thr);
-	}
-
 		mutex_exit(&srv_sys.tasks_mutex);
-
-	if (thr != NULL) {
-
 		que_run_threads(thr);
-
-		my_atomic_addlint(
-			&purge_sys.n_completed, 1);
+		my_atomic_addlint(&purge_sys.n_completed, 1);
+		return true;
 	}
 
-	return(thr != NULL);
+	ut_ad(UT_LIST_GET_LEN(srv_sys.tasks) == 0);
+	mutex_exit(&srv_sys.tasks_mutex);
+	return false;
 }
 
 /*********************************************************************//**
@@ -2781,8 +2768,8 @@ DECLARE_THREAD(srv_purge_coordinator_thread)(
 
 		if (srv_shutdown_state == SRV_SHUTDOWN_NONE
 		    && srv_undo_sources
-		    && (purge_sys.state == PURGE_STATE_STOP
-			|| n_total_purged == 0)) {
+		    && (n_total_purged == 0
+			|| purge_sys.state == PURGE_STATE_STOP)) {
 
 			srv_purge_coordinator_suspend(slot, rseg_history_len);
 		}

storage/innobase/trx/trx0purge.cc

@@ -1510,10 +1510,9 @@ static
 void
 trx_purge_wait_for_workers_to_complete()
 {
-	ulint		n_submitted = purge_sys.n_submitted;
-
 	/* Ensure that the work queue empties out. */
-	while ((ulint) my_atomic_loadlint(&purge_sys.n_completed) != n_submitted) {
+	while (my_atomic_loadlint(&purge_sys.n_completed)
+	       != purge_sys.n_submitted) {
 
 		if (srv_get_task_queue_length() > 0) {
 			srv_release_threads(SRV_WORKER, 1);
@@ -1522,9 +1521,6 @@ trx_purge_wait_for_workers_to_complete()
 		os_thread_yield();
 	}
 
-	/* None of the worker threads should be doing any work. */
-	ut_a(purge_sys.n_submitted == purge_sys.n_completed);
-
 	/* There should be no outstanding tasks as long
 	as the worker threads are active. */
 	ut_a(srv_get_task_queue_length() == 0);
@@ -1548,7 +1544,8 @@ trx_purge(
 	srv_dml_needed_delay = trx_purge_dml_delay();
 
 	/* The number of tasks submitted should be completed. */
-	ut_a(purge_sys.n_submitted == purge_sys.n_completed);
+	ut_a(purge_sys.n_submitted
+	     == my_atomic_loadlint(&purge_sys.n_completed));
 
 	rw_lock_x_lock(&purge_sys.latch);
 	trx_sys.clone_oldest_view();
@@ -1562,35 +1559,16 @@ trx_purge(
 
 	/* Fetch the UNDO recs that need to be purged. */
 	n_pages_handled = trx_purge_attach_undo_recs(n_purge_threads);
+	purge_sys.n_submitted += n_purge_threads;
 
-	/* Do we do an asynchronous purge or not ? */
-	if (n_purge_threads > 1) {
-		ulint	i = 0;
-
-		/* Submit the tasks to the work queue. */
-		for (i = 0; i < n_purge_threads - 1; ++i) {
-			thr = que_fork_scheduler_round_robin(
-				purge_sys.query, thr);
-
-			ut_a(thr != NULL);
-
+	/* Submit tasks to workers queue if using multi-threaded purge. */
+	for (ulint i = n_purge_threads; --i; ) {
+		thr = que_fork_scheduler_round_robin(purge_sys.query, thr);
+		ut_a(thr);
 		srv_que_task_enqueue_low(thr);
 	}
 
 	thr = que_fork_scheduler_round_robin(purge_sys.query, thr);
-		ut_a(thr != NULL);
-
-		purge_sys.n_submitted += n_purge_threads - 1;
-
-		goto run_synchronously;
-
-	/* Do it synchronously. */
-	} else {
-		thr = que_fork_scheduler_round_robin(purge_sys.query, NULL);
-		ut_ad(thr);
-
-run_synchronously:
-		++purge_sys.n_submitted;
 
 	que_run_threads(thr);
 
@@ -1599,9 +1577,9 @@ trx_purge(
 	if (n_purge_threads > 1) {
 		trx_purge_wait_for_workers_to_complete();
 	}
-	}
 
-	ut_a(purge_sys.n_submitted == purge_sys.n_completed);
+	ut_a(purge_sys.n_submitted
+	     == my_atomic_loadlint(&purge_sys.n_completed));
 
 	if (truncate) {
 		trx_purge_truncate_history();
@@ -1653,7 +1631,6 @@ trx_purge_stop(void)
 	case PURGE_STATE_STOP:
 		ut_ad(srv_n_purge_threads > 0);
 		++purge_sys.n_stop;
-		purge_sys.state = PURGE_STATE_STOP;
 		if (!purge_sys.running) {
 			goto unlock;
 		}