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.c

@@ -2103,7 +2103,15 @@ buf_flush_stat_update(void)
 	ib_uint64_t		lsn;
 	ulint			n_flushed;
 
-	lsn = log_get_lsn();
+	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 (lsn == 0) {
+		return;
+	}
+
 	if (buf_flush_stat_cur.redo == 0) {
 		/* First time around. Just update the current LSN
 		and return. */

storage/innobase/include/log0log.ic

@@ -419,14 +419,14 @@ 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;
 
 		mutex_exit(&(log_sys->mutex));
+	}
 
 	return(lsn);
 }

storage/xtradb/buf/buf0flu.c

@@ -2214,7 +2214,15 @@ buf_flush_stat_update(void)
 	ib_uint64_t		lsn;
 	ulint			n_flushed;
 
-	lsn = log_get_lsn();
+	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 (lsn == 0) {
+		return;
+	}
+
 	if (buf_flush_stat_cur.redo == 0) {
 		/* First time around. Just update the current LSN
 		and return. */

storage/xtradb/include/log0log.ic

@@ -434,14 +434,14 @@ 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;
 
 		mutex_exit(&(log_sys->mutex));
+	}
 
 	return(lsn);
 }