MDEV-26010 fixup: Use acquire/release memory order

In commit 5f22511e35674ecc376f4c56217ee2d78f92c772 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().

1 files changed, 3 insertions, 2 deletions

diff --git a/include/my_atomic_wrapper.h b/include/my_atomic_wrapper.h
index d57559d360d..c5820b4f5b6 100644
--- a/include/my_atomic_wrapper.h
+++ b/include/my_atomic_wrapper.h
@@ -43,9 +43,10 @@ public:
 
   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 val)
-  { m.store(val, std::memory_order_relaxed); return val; }
+  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); }