MDEV-7100: InnoDB error monitor might unnecessary wait log_sys mutex

Analysis: InnoDB error monitor is responsible to call every second
sync_arr_wake_threads_if_sema_free() to wake up possible hanging
threads if they are missed in mutex_signal_object. This is not
possible if error monitor itself is on mutex/semaphore wait. We
should avoid all unnecessary mutex/semaphore waits on error monitor.
Currently error monitor calls function buf_flush_stat_update()
that calls log_get_lsn() function and there we will try to get
log_sys mutex. Better, solution for error monitor is that in
buf_flush_stat_update() we will try to get lsn with
mutex_enter_nowait() and if we did not get mutex do not update
the stats.

Fix: Use log_get_lsn_nowait() function on buf_flush_stat_update()
function. If returned lsn is 0, we do not update flush stats.
log_get_lsn_nowait() will use mutex_enter_nowait() and if
we get mutex we return a correct lsn if not we return 0.

storage/innobase/buf/buf0flu.cc

@@ -2288,7 +2288,14 @@ page_cleaner_flush_pages_if_needed(void)
 	ulint			pct_total = 0;
 	int			age_factor = 0;
 
-	cur_lsn = log_get_lsn();
+	cur_lsn = log_get_lsn_nowait();
+
+	/* log_get_lsn_nowait tries to get log_sys->mutex with
+	mutex_enter_nowait, if this does not succeed function
+	returns 0, do not use that value to update stats. */
+	if (cur_lsn == 0) {
+		return(0);
+	}
 
 	if (prev_lsn == 0) {
 		/* First time around. */

storage/innobase/include/log0log.ic

@@ -442,14 +442,14 @@ lsn_t
 log_get_lsn_nowait(void)
 /*=============*/
 {
-	lsn_t	lsn;
+	lsn_t	lsn=0;
 
-        if (mutex_enter_nowait(&(log_sys->mutex)))
-          return 0;
+	if (!mutex_enter_nowait(&(log_sys->mutex))) {
 
-	lsn = log_sys->lsn;
+		lsn = log_sys->lsn;
 
-	mutex_exit(&(log_sys->mutex));
+		mutex_exit(&(log_sys->mutex));
+	}
 
 	return(lsn);
 }

storage/xtradb/buf/buf0flu.cc

@@ -2474,7 +2474,14 @@ page_cleaner_flush_pages_if_needed(void)
 	ulint			pct_total = 0;
 	int			age_factor = 0;
 
-	cur_lsn = log_get_lsn();
+	cur_lsn = log_get_lsn_nowait();
+
+	/* log_get_lsn_nowait tries to get log_sys->mutex with
+	mutex_enter_nowait, if this does not succeed function
+	returns 0, do not use that value to update stats. */
+	if (cur_lsn == 0) {
+		return(0);
+	}
 
 	if (prev_lsn == 0) {
 		/* First time around. */

storage/xtradb/include/log0log.ic

@@ -494,14 +494,14 @@ lsn_t
 log_get_lsn_nowait(void)
 /*=============*/
 {
-	lsn_t	lsn;
+	lsn_t	lsn=0;
 
-        if (mutex_enter_nowait(&(log_sys->mutex)))
-          return 0;
+	if (!mutex_enter_nowait(&(log_sys->mutex))) {
 
-	lsn = log_sys->lsn;
+		lsn = log_sys->lsn;
 
-	mutex_exit(&(log_sys->mutex));
+		mutex_exit(&(log_sys->mutex));
+	}
 
 	return(lsn);
 }