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, 5 deletions

diff --git a/storage/innobase/buf/buf0buf.cc b/storage/innobase/buf/buf0buf.cc
index 72dd2cdc4ab..bbbb67a69df 100644
--- a/storage/innobase/buf/buf0buf.cc
+++ b/storage/innobase/buf/buf0buf.cc
@@ -1333,21 +1333,19 @@ inline const buf_block_t *buf_pool_t::chunk_t::not_freed() const
       /* Skip blocks that are not being used for file pages. */
       break;
     case BUF_BLOCK_FILE_PAGE:
+      const lsn_t lsn= block->page.oldest_modification();
+
       if (srv_read_only_mode)
       {
         /* The page cleaner is disabled in read-only mode.  No pages
         can be dirtied, so all of them must be clean. */
-        ut_d(lsn_t oldest_modification= block->page.oldest_modification());
-        ut_ad(oldest_modification == 0 ||
-              oldest_modification == recv_sys.recovered_lsn ||
+        ut_ad(lsn == 0 || lsn == recv_sys.recovered_lsn ||
               srv_force_recovery == SRV_FORCE_NO_LOG_REDO);
         ut_ad(!block->page.buf_fix_count());
         ut_ad(block->page.io_fix() == BUF_IO_NONE);
         break;
       }
 
-      const lsn_t lsn= block->page.oldest_modification();
-
       if (fsp_is_system_temporary(block->page.id().space()))
       {
         ut_ad(lsn == 0 || lsn == 2);