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.

1 files changed, 5 insertions, 5 deletions

diff --git a/storage/xtradb/include/log0log.ic b/storage/xtradb/include/log0log.ic
index 853027daa7e..ff5a83be249 100644
--- a/storage/xtradb/include/log0log.ic
+++ b/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);
 }