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.

Author: mysqlonarm

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 @@ rw_lock_s_lock_spin(
 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 @@ rw_lock_x_lock_func(
 
 /* 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 @@ rw_lock_x_lock_func(
 }
 
 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_tcount_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)) {
@@ -765,24 +781,22 @@ rw_lock_sx_lock_func(
 }
 
 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) {
 
 ut_delay(srv_spin_wait_delay);
 i++;
 }
 
-spin_count += i;
+spin_count += lint(i - j);
 
 if (i >= srv_n_spin_wait_rounds) {
 
@@ -814,7 +828,6 @@ rw_lock_sx_lock_func(
 }
 
 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;