MDEV-33515 fixup: Clarify mtr_t::spin_wait_delay

innodb_log_spin_wait_delay_update(): Always acquire log_sys.latch
to protect the change of mtr_t::spin_wait_delay.

log_t::lock_lsn(): In the general case, actually use
mtr_t::spin_wait_delay as it was intended. In the x86 specific
log_t::lock_lsn_bts() we used mtr_t::spin_wait_delay.

storage/innobase/handler/ha_innodb.cc

@@ -18478,22 +18478,13 @@ static void innodb_log_file_size_update(THD *thd, st_mysql_sys_var*,
   mysql_mutex_lock(&LOCK_global_system_variables);
 }
 
-static void innodb_log_spin_wait_delay_update(THD *thd, st_mysql_sys_var*,
-                                              void *var, const void *save)
+static void innodb_log_spin_wait_delay_update(THD *, st_mysql_sys_var*,
+                                              void *, const void *save)
 {
-  ut_ad(var == &mtr_t::spin_wait_delay);
-
-  unsigned delay= *static_cast<const unsigned*>(save);
-
-  if (!delay != !mtr_t::spin_wait_delay)
-  {
   log_sys.latch.wr_lock(SRW_LOCK_CALL);
-    mtr_t::spin_wait_delay= delay;
+  mtr_t::spin_wait_delay= *static_cast<const unsigned*>(save);
   mtr_t::finisher_update();
   log_sys.latch.wr_unlock();
-  }
-  else
-    mtr_t::spin_wait_delay= delay;
 }
 
 /** Update innodb_status_output or innodb_status_output_locks,

storage/innobase/include/mtr0mtr.h

@@ -715,8 +715,7 @@ struct mtr_t {
   { return finisher(this, len); }
 public:
   /** Poll interval in log_sys.lock_lsn(); 0 to use log_sys.lsn_lock.
-  Protected by LOCK_global_system_variables; changes to and from 0
-  are additionally protected by exclusive log_sys.latch. */
+  Protected by LOCK_global_system_variables and log_sys.latch. */
   static unsigned spin_wait_delay;
   /** Update finisher when spin_wait_delay is changing to or from 0. */
   static void finisher_update();

storage/innobase/mtr/mtr0mtr.cc

@@ -962,7 +962,7 @@ size_t log_t::lock_lsn() noexcept
   size_t b= buf_free.fetch_or(buf_free_LOCK, std::memory_order_acquire);
   if (b & buf_free_LOCK)
   {
-    const size_t m= my_cpu_relax_multiplier * srv_spin_wait_delay / 32;
+    const size_t m= mtr_t::spin_wait_delay;
     constexpr size_t DELAY= 10, MAX_ITERATIONS= 10;
     for (size_t delay_count= DELAY, delay_iterations= 1;
          ((b= buf_free.load(std::memory_order_relaxed)) & buf_free_LOCK) ||