mariadb/include/my_atomic_wrapper.h
Marko Mäkelä 759deaa0a2 MDEV-26010 fixup: Use acquire/release memory order
In commit 5f22511e35 we depend on
Total Store Ordering. For correct operation on ISAs that implement
weaker memory ordering, we must explicitly use release/acquire stores
and loads on buf_page_t::oldest_modification_ to prevent a race condition
when buf_page_t::list does not happen to be on the same cache line.

buf_page_t::clear_oldest_modification(): Assert that the block is
not in buf_pool.flush_list, and use std::memory_order_release.

buf_page_t::oldest_modification_acquire(): Read oldest_modification_
with std::memory_order_acquire. In this way, if the return value is 0,
the caller may safely assume that it will not observe the buf_page_t
as being in buf_pool.flush_list, even if it is not holding
buf_pool.flush_list_mutex.

buf_flush_relocate_on_flush_list(), buf_LRU_free_page():
Invoke buf_page_t::oldest_modification_acquire().
2021-06-26 11:16:40 +03:00

68 lines
2.9 KiB
C++

/* Copyright (c) 2020, 2021, 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
the Free Software Foundation; version 2 of the License.
This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
GNU General Public License for more details.
You should have received a copy of the GNU General Public License
along with this program; if not, write to the Free Software
Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1335 USA */
#pragma once
#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() {}
Type load(std::memory_order o= std::memory_order_relaxed) const
{ return m.load(o); }
void store(Type i, std::memory_order o= std::memory_order_relaxed)
{ m.store(i, o); }
operator Type() const { return m.load(); }
Type operator=(const Type i) { store(i); return i; }
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); }
Type fetch_xor(const Type i, std::memory_order o= std::memory_order_relaxed)
{ return m.fetch_xor(i, o); }
Type fetch_and(const Type i, std::memory_order o= std::memory_order_relaxed)
{ return m.fetch_and(i, o); }
Type fetch_or(const Type i, std::memory_order o= std::memory_order_relaxed)
{ return m.fetch_or(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 */