MDEV-22544: Inconsistent and Incorrect rw-lock stats

- There are multiple inconsistency and incorrect way in which rw-lock
  stats are calculated.

- shared rw-lock stats:
  "rounds" counter is incremented only once for N rounds done
  in spin-cycle.

- all rw-lock stats:
  If the spin-cycle is short-circuited then attempts are re-counted.
  [If spin-cycle is interrupted, before it completes
   srv_n_spin_wait_rounds (default 30) rounds, spin_count is incremented
   to consider this. If thread resumes spin-cycle (due to unavailability
   of the locks) and is again interrupted or completed, spin_count
   is again incremented with the total count, failing to adjust the
   previous attempt increment].

- s/x rw-lock stats:
  spin_loop counter is not incremented at-all instead it is projected
  as 0 (in show engine output) and division to calculate spin-round per
  spin-loop is adjusted.
  As per the original semantics spin_loop counter should be incremented
  once per spin_loop execution.

- sx rw-lock stats:
  sx locks increments spin_loop counter but instead of incrementing it
  once for a spin_loop invocation it does it multiple times based on how
  many time spin_loop flow is repeated for same instance post os-wait.

1 files changed, 22 insertions, 9 deletions

diff --git a/storage/innobase/sync/sync0rw.cc b/storage/innobase/sync/sync0rw.cc
index df710a53cf6..b46126bde58 100644
--- a/storage/innobase/sync/sync0rw.cc
+++ b/storage/innobase/sync/sync0rw.cc
@@ -293,10 +293,13 @@ rw_lock_s_lock_spin(
 
 	ut_ad(rw_lock_validate(lock));
 
+	rw_lock_stats.rw_s_spin_wait_count.inc();
+
 lock_loop:
 
 	/* Spin waiting for the writer field to become free */
 	HMT_low();
+	ulint j = i;
 	while (i < srv_n_spin_wait_rounds && lock->lock_word <= 0) {
 		ut_delay(srv_spin_wait_delay);
 		i++;
@@ -307,7 +310,7 @@ lock_loop:
 		os_thread_yield();
 	}
 
-	++spin_count;
+	spin_count += lint(i - j);
 
 	/* We try once again to obtain the lock */
 	if (rw_lock_s_lock_low(lock, pass, file_name, line)) {
@@ -428,7 +431,7 @@ rw_lock_x_lock_wait_func(
 		HMT_medium();
 
 		/* If there is still a reader, then go to sleep.*/
-		++n_spins;
+		n_spins += i;
 
 		sync_cell_t*	cell;
 
@@ -654,6 +657,12 @@ rw_lock_x_lock_func(
 	ut_ad(rw_lock_validate(lock));
 	ut_ad(!rw_lock_own(lock, RW_LOCK_S));
 
+	if (rw_lock_x_lock_low(lock, pass, file_name, line)) {
+		/* Locking succeeded */
+		return;
+	}
+	rw_lock_stats.rw_x_spin_wait_count.inc();
+
 lock_loop:
 
 	if (rw_lock_x_lock_low(lock, pass, file_name, line)) {
@@ -673,6 +682,7 @@ lock_loop:
 
 		/* Spin waiting for the lock_word to become free */
 		HMT_low();
+		ulint j = i;
 		while (i < srv_n_spin_wait_rounds
 		       && lock->lock_word <= X_LOCK_HALF_DECR) {
 
@@ -681,7 +691,7 @@ lock_loop:
 		}
 
 		HMT_medium();
-		spin_count += i;
+		spin_count += lint(i - j);
 
 		if (i >= srv_n_spin_wait_rounds) {
 
@@ -749,11 +759,17 @@ rw_lock_sx_lock_func(
 	sync_array_t*	sync_arr;
 	ulint		spin_count = 0;
 	uint64_t	count_os_wait = 0;
-	ulint		spin_wait_count = 0;
 
 	ut_ad(rw_lock_validate(lock));
 	ut_ad(!rw_lock_own(lock, RW_LOCK_S));
 
+	if (rw_lock_sx_lock_low(lock, pass, file_name, line)) {
+		/* Locking succeeded */
+		return;
+	}
+
+	rw_lock_stats.rw_sx_spin_wait_count.inc();
+
 lock_loop:
 
 	if (rw_lock_sx_lock_low(lock, pass, file_name, line)) {
@@ -765,16 +781,14 @@ lock_loop:
 		}
 
 		rw_lock_stats.rw_sx_spin_round_count.add(spin_count);
-		rw_lock_stats.rw_sx_spin_wait_count.add(spin_wait_count);
 
 		/* Locking succeeded */
 		return;
 
 	} else {
 
-		++spin_wait_count;
-
 		/* Spin waiting for the lock_word to become free */
+		ulint j = i;
 		while (i < srv_n_spin_wait_rounds
 		       && lock->lock_word <= X_LOCK_HALF_DECR) {
 
@@ -782,7 +796,7 @@ lock_loop:
 			i++;
 		}
 
-		spin_count += i;
+		spin_count += lint(i - j);
 
 		if (i >= srv_n_spin_wait_rounds) {
 
@@ -814,7 +828,6 @@ lock_loop:
 		}
 
 		rw_lock_stats.rw_sx_spin_round_count.add(spin_count);
-		rw_lock_stats.rw_sx_spin_wait_count.add(spin_wait_count);
 
 		/* Locking succeeded */
 		return;