MDEV-22456 after-merge fix: introduce Atomic_relaxed

In the merge 9e6e43551f
we made Atomic_counter a more generic wrapper of std::atomic
so that dict_index_t would support the implicit assignment operator.

It is better to revert the changes to Atomic_counter and
instead introduce Atomic_relaxed as a generic wrapper to std::atomic.

Unlike Atomic_counter, we will not define operator++, operator+=
or similar, because we want to make the operations more explicit
in the users of Atomic_wrapper, because unlike loads and stores,
atomic read-modify-write operations always incur some overhead.

include/my_atomic.h

@@ -2,6 +2,7 @@
 #define MY_ATOMIC_INCLUDED
 
 /* Copyright (c) 2006, 2010, Oracle and/or its affiliates. All rights reserved.
+   Copyright (c) 2018, 2020, MariaDB
 
    This program is free software; you can redistribute it and/or modify
    it under the terms of the GNU General Public License as published by
@@ -169,4 +170,48 @@
   my_atomic_casptr((P), (E), (D))
 #endif
 
+#ifdef __cplusplus
+#include <atomic>
+/**
+  A wrapper for std::atomic, defaulting to std::memory_order_relaxed.
+
+  When it comes to atomic loads or stores at std::memory_order_relaxed
+  on IA-32 or AMD64, this wrapper is only introducing some constraints
+  to the C++ compiler, to prevent some optimizations of loads or
+  stores.
+
+  On POWER and ARM, atomic loads and stores involve different instructions
+  from normal loads and stores and will thus incur some overhead.
+
+  Because atomic read-modify-write operations will always incur
+  overhead, we intentionally do not define
+  operator++(), operator--(), operator+=(), operator-=(), or similar,
+  to make the overhead stand out in the users of this code.
+*/
+template <typename Type> class Atomic_relaxed
+{
+  std::atomic<Type> m;
+public:
+  Atomic_relaxed(const Atomic_relaxed<Type> &rhs)
+  { m.store(rhs, std::memory_order_relaxed); }
+  Atomic_relaxed(Type val) : m(val) {}
+  Atomic_relaxed() {}
+
+  operator Type() const { return m.load(std::memory_order_relaxed); }
+  Type operator=(const Type val)
+  { m.store(val, std::memory_order_relaxed); return val; }
+  Type operator=(const Atomic_relaxed<Type> &rhs) { return *this= Type{rhs}; }
+  Type fetch_add(const Type i, std::memory_order o= std::memory_order_relaxed)
+  { return m.fetch_add(i, o); }
+  Type fetch_sub(const Type i, std::memory_order o= std::memory_order_relaxed)
+  { return m.fetch_sub(i, o); }
+  bool compare_exchange_strong(Type& i1, const Type i2,
+                               std::memory_order o1= std::memory_order_relaxed,
+                               std::memory_order o2= std::memory_order_relaxed)
+  { return m.compare_exchange_strong(i1, i2, o1, o2); }
+  Type exchange(const Type i, std::memory_order o= std::memory_order_relaxed)
+  { return m.exchange(i, o); }
+};
+#endif /* __cplusplus */
+
 #endif /* MY_ATOMIC_INCLUDED */

include/my_counter.h

@@ -1,7 +1,7 @@
 #ifndef MY_COUNTER_H_INCLUDED
 #define MY_COUNTER_H_INCLUDED
 /*
-   Copyright (C) 2018, 2020, MariaDB
+   Copyright (C) 2018 MariaDB Foundation
 
    This program is free software; you can redistribute it and/or modify
    it under the terms of the GNU General Public License as published by
@@ -45,16 +45,5 @@ public:
   operator Type() const { return m_counter.load(std::memory_order_relaxed); }
   Type operator=(const Type val)
   { m_counter.store(val, std::memory_order_relaxed); return val; }
-  Type operator=(const Atomic_counter<Type> &rhs) { return *this= Type{rhs}; }
-
-  Type fetch_add(const Type i, std::memory_order m)
-  { return m_counter.fetch_add(i, m); }
-  Type fetch_sub(const Type i, std::memory_order m)
-  { return m_counter.fetch_sub(i, m); }
-  bool compare_exchange_strong(Type& i1, const Type i2,
-                               std::memory_order m1, std::memory_order m2)
-  { return m_counter.compare_exchange_strong(i1, i2, m1, m2); }
-  Type exchange(const Type i, std::memory_order m)
-  { return m_counter.exchange(i, m); }
 };
 #endif /* MY_COUNTER_H_INCLUDED */

storage/innobase/dict/dict0dict.cc

@@ -6158,10 +6158,7 @@ dict_index_zip_pad_update(
 		beyond max pad size. */
 		if (info->pad + ZIP_PAD_INCR
 		    < (srv_page_size * zip_pad_max) / 100) {
-			/* Use atomics even though we have the mutex.
-			This is to ensure that we are able to read
-			info->pad atomically. */
-			info->pad += ZIP_PAD_INCR;
+			info->pad.fetch_add(ZIP_PAD_INCR);
 
 			MONITOR_INC(MONITOR_PAD_INCREMENTS);
 		}
@@ -6178,11 +6175,7 @@ dict_index_zip_pad_update(
 		padding. */
 		if (info->n_rounds >= ZIP_PAD_SUCCESSFUL_ROUND_LIMIT
 		    && info->pad > 0) {
-
-			/* Use atomics even though we have the mutex.
-			This is to ensure that we are able to read
-			info->pad atomically. */
-			info->pad -= ZIP_PAD_INCR;
+			info->pad.fetch_sub(ZIP_PAD_INCR);
 
 			info->n_rounds = 0;
 

storage/innobase/include/dict0mem.h

@@ -931,7 +931,7 @@ an uncompressed page should be left as padding to avoid compression
 failures. This estimate is based on a self-adapting heuristic. */
 struct zip_pad_info_t {
 	SysMutex	mutex;	/*!< mutex protecting the info */
-	Atomic_counter<ulint>
+	Atomic_relaxed<ulint>
 			pad;	/*!< number of bytes used as pad */
 	ulint		success;/*!< successful compression ops during
 				current round */
@@ -1107,10 +1107,10 @@ struct dict_index_t {
 	/* @} */
 private:
   /** R-tree split sequence number */
-  Atomic_counter<node_seq_t> rtr_ssn;
+  Atomic_relaxed<node_seq_t> rtr_ssn;
 public:
   void set_ssn(node_seq_t ssn) { rtr_ssn= ssn; }
-  node_seq_t assign_ssn() { return ++rtr_ssn; }
+  node_seq_t assign_ssn() { return rtr_ssn.fetch_add(1) + 1; }
   node_seq_t ssn() const { return rtr_ssn; }
 
 	rtr_info_track_t*

storage/innobase/include/sync0rw.h

@@ -569,10 +569,10 @@ struct rw_lock_t
 #endif /* UNIV_DEBUG */
 {
   /** Holds the state of the lock. */
-  Atomic_counter<int32_t> lock_word;
+  Atomic_relaxed<int32_t> lock_word;
 
-  /** 1: there are waiters */
-  Atomic_counter<uint32_t> waiters;
+  /** 0=no waiters, 1=waiters for X or SX lock exist */
+  Atomic_relaxed<uint32_t> waiters;
 
 	/** number of granted SX locks. */
 	volatile ulint	sx_recursive;

storage/innobase/include/sync0rw.ic

@@ -355,16 +355,15 @@ rw_lock_s_unlock_func(
 	ut_d(rw_lock_remove_debug_info(lock, pass, RW_LOCK_S));
 
 	/* Increment lock_word to indicate 1 less reader */
-	int32_t lock_word = ++lock->lock_word;
+	int32_t lock_word = lock->lock_word.fetch_add(1);
 
-	if (lock_word == 0 || lock_word == -X_LOCK_HALF_DECR) {
+	if (lock_word == -1 || lock_word == -X_LOCK_HALF_DECR - 1) {
 		/* wait_ex waiter exists. It may not be asleep, but we signal
 		anyway. We do not wake other waiters, because they can't
 		exist without wait_ex waiter and wait_ex waiter goes first.*/
 		os_event_set(lock->wait_ex_event);
 		sync_array_object_signalled();
 	} else {
-		ut_ad(--lock_word);
 		ut_ad(lock_word > -X_LOCK_DECR);
 		ut_ad(lock_word < X_LOCK_DECR);
 	}
@@ -414,11 +413,11 @@ rw_lock_x_unlock_func(
 	} else if (lock_word == -X_LOCK_DECR
 		   || lock_word == -(X_LOCK_DECR + X_LOCK_HALF_DECR)) {
 		/* There are 2 x-locks */
-		lock->lock_word += X_LOCK_DECR;
+		lock->lock_word.fetch_add(X_LOCK_DECR);
 	} else {
 		/* There are more than 2 x-locks. */
 		ut_ad(lock_word < -X_LOCK_DECR);
-		lock->lock_word++;
+		lock->lock_word.fetch_add(1);
 	}
 
 	ut_ad(rw_lock_validate(lock));
@@ -470,7 +469,7 @@ rw_lock_sx_unlock_func(
 			/* still has x-lock */
 			ut_ad(lock_word == -X_LOCK_HALF_DECR ||
 			      lock_word <= -(X_LOCK_DECR + X_LOCK_HALF_DECR));
-			lock->lock_word += X_LOCK_HALF_DECR;
+			lock->lock_word.fetch_add(X_LOCK_HALF_DECR);
 		}
 	}
 

storage/innobase/sync/sync0rw.cc

@@ -528,10 +528,10 @@ rw_lock_x_lock_low(
 				exists. Add another. */
 				if (lock_word == 0
 				    || lock_word == -X_LOCK_HALF_DECR) {
-					lock->lock_word -= X_LOCK_DECR;
+					lock->lock_word.fetch_sub(X_LOCK_DECR);
 				} else {
 					ut_ad(lock_word <= -X_LOCK_DECR);
-					lock->lock_word--;
+					lock->lock_word.fetch_sub(1);
 				}
 			}