MDEV-26200 buf_pool.flush_list corrupted by buffer pool resizing or ROW_FORMAT=COMPRESSED

The lazy deletion of clean blocks from buf_pool.flush_list that was
introduced in commit 6441bc61 (MDEV-25113)
introduced a race condition around the function
buf_flush_relocate_on_flush_list().

The test innodb_zip.wl5522_debug_zip as well as the buffer pool
resizing tests would occasionally fail in debug builds due to
buf_pool.flush_list.count disagreeing with the actual length of the
doubly-linked list.

The safe procedure for relocating a block in buf_pool.flush_list should be
as follows, now that we implement lazy deletion from buf_pool.flush_list:

1. Acquire buf_pool.mutex.
2. Acquire the exclusive buf_pool.page_hash.latch.
3. Acquire buf_pool.flush_list_mutex.
4. Copy the block descriptor.
5. Invoke buf_flush_relocate_on_flush_list().
6. Release buf_pool.flush_list_mutex.

buf_flush_relocate_on_flush_list(): Assert that
buf_pool.flush_list_mutex is being held. Invoke
buf_page_t::oldest_modification() only once, using
std::memory_order_relaxed, now that the mutex protects us.

buf_LRU_free_page(), buf_LRU_block_remove_hashed(): Avoid
an unlock-lock cycle on hash_lock. (We must not acquire hash_lock
while already holding buf_pool.flush_list_mutex, because that
could lead to a deadlock due to latching order violation.)

storage/innobase/buf/buf0buf.cc

@@ -1566,6 +1566,7 @@ inline bool buf_pool_t::realloc(buf_block_t *block)
 	if (block->page.can_relocate()) {
 		memcpy_aligned<OS_FILE_LOG_BLOCK_SIZE>(
 			new_block->frame, block->frame, srv_page_size);
+		mysql_mutex_lock(&buf_pool.flush_list_mutex);
 		new (&new_block->page) buf_page_t(block->page);
 
 		/* relocate LRU list */
@@ -1625,6 +1626,7 @@ inline bool buf_pool_t::realloc(buf_block_t *block)
 			buf_flush_relocate_on_flush_list(&block->page,
 							 &new_block->page);
 		}
+		mysql_mutex_unlock(&buf_pool.flush_list_mutex);
 		block->page.set_corrupt_id();
 
 		/* set other flags of buf_block_t */
@@ -3131,12 +3133,14 @@ buf_page_get_low(
 		/* Note: this is the uncompressed block and it is not
 		accessible by other threads yet because it is not in
 		any list or hash table */
+		mysql_mutex_lock(&buf_pool.flush_list_mutex);
 		buf_relocate(bpage, &block->page);
 
 		/* Set after buf_relocate(). */
 		block->page.set_buf_fix_count(1);
 
 		buf_flush_relocate_on_flush_list(bpage, &block->page);
+		mysql_mutex_unlock(&buf_pool.flush_list_mutex);
 
 		/* Buffer-fix, I/O-fix, and X-latch the block
 		for the duration of the decompression.
@@ -3646,8 +3650,10 @@ buf_page_create(fil_space_t *space, uint32_t offset,
       }
 
       rw_lock_x_lock(&free_block->lock);
+      mysql_mutex_lock(&buf_pool.flush_list_mutex);
       buf_relocate(&block->page, &free_block->page);
       buf_flush_relocate_on_flush_list(&block->page, &free_block->page);
+      mysql_mutex_unlock(&buf_pool.flush_list_mutex);
 
       free_block->page.set_state(BUF_BLOCK_FILE_PAGE);
       buf_unzip_LRU_add_block(free_block, FALSE);

storage/innobase/buf/buf0flu.cc

@@ -286,43 +286,29 @@ buf_flush_relocate_on_flush_list(
 {
 	buf_page_t*	prev;
 
-	mysql_mutex_assert_owner(&buf_pool.mutex);
+	mysql_mutex_assert_owner(&buf_pool.flush_list_mutex);
 	ut_ad(!fsp_is_system_temporary(bpage->id().space()));
 
-	const lsn_t lsn = bpage->oldest_modification_acquire();
+	const lsn_t lsn = bpage->oldest_modification();
 
 	if (!lsn) {
 		return;
 	}
 
 	ut_ad(lsn == 1 || lsn > 2);
-
-	mysql_mutex_lock(&buf_pool.flush_list_mutex);
-
-	/* FIXME: Can we avoid holding buf_pool.mutex here? */
 	ut_ad(dpage->oldest_modification() == lsn);
 
-	if (ut_d(const lsn_t o_lsn =) bpage->oldest_modification()) {
-		ut_ad(o_lsn == lsn);
+	/* Important that we adjust the hazard pointer before removing
+	the bpage from the flush list. */
+	buf_pool.flush_hp.adjust(bpage);
 
-		/* Important that we adjust the hazard pointer before removing
-		the bpage from the flush list. */
-		buf_pool.flush_hp.adjust(bpage);
+	prev = UT_LIST_GET_PREV(list, bpage);
+	UT_LIST_REMOVE(buf_pool.flush_list, bpage);
 
-		prev = UT_LIST_GET_PREV(list, bpage);
-		UT_LIST_REMOVE(buf_pool.flush_list, bpage);
-
-		bpage->clear_oldest_modification();
-	} else {
-		/* bpage was removed from buf_pool.flush_list
-		since we last checked, and before we acquired
-		buf_pool.flush_list_mutex. */
-		goto was_clean;
-	}
+	bpage->clear_oldest_modification();
 
 	if (lsn == 1) {
 		buf_pool.stat.flush_list_bytes -= dpage->physical_size();
-was_clean:
 		dpage->list.prev = nullptr;
 		dpage->list.next = nullptr;
 		dpage->clear_oldest_modification();
@@ -334,7 +320,6 @@ buf_flush_relocate_on_flush_list(
 	}
 
 	ut_d(buf_flush_validate_low());
-	mysql_mutex_unlock(&buf_pool.flush_list_mutex);
 }
 
 /** Complete write of a file page from buf_pool.

storage/innobase/buf/buf0lru.cc

@@ -845,6 +845,7 @@ bool buf_LRU_free_page(buf_page_t *bpage, bool zip)
 	} else if (bpage->state() == BUF_BLOCK_FILE_PAGE) {
 		b = buf_page_alloc_descriptor();
 		ut_a(b);
+		mysql_mutex_lock(&buf_pool.flush_list_mutex);
 		new (b) buf_page_t(*bpage);
 		b->set_state(BUF_BLOCK_ZIP_PAGE);
 	}
@@ -859,6 +860,8 @@ bool buf_LRU_free_page(buf_page_t *bpage, bool zip)
 	ut_ad(bpage->can_relocate());
 
 	if (!buf_LRU_block_remove_hashed(bpage, id, hash_lock, zip)) {
+		ut_ad(!b);
+		mysql_mutex_assert_not_owner(&buf_pool.flush_list_mutex);
 		return(true);
 	}
 
@@ -872,8 +875,6 @@ bool buf_LRU_free_page(buf_page_t *bpage, bool zip)
 	if (UNIV_LIKELY_NULL(b)) {
 		buf_page_t*	prev_b	= UT_LIST_GET_PREV(LRU, b);
 
-		hash_lock->write_lock();
-
 		ut_ad(!buf_pool.page_hash_get_low(id, fold));
 		ut_ad(b->zip_size());
 
@@ -940,6 +941,7 @@ bool buf_LRU_free_page(buf_page_t *bpage, bool zip)
 		}
 
 		buf_flush_relocate_on_flush_list(bpage, b);
+		mysql_mutex_unlock(&buf_pool.flush_list_mutex);
 
 		bpage->zip.data = nullptr;
 
@@ -950,6 +952,8 @@ bool buf_LRU_free_page(buf_page_t *bpage, bool zip)
 		hash_lock. */
 		b->set_io_fix(BUF_IO_PIN);
 		hash_lock->write_unlock();
+	} else if (!zip) {
+		hash_lock->write_unlock();
 	}
 
 	buf_block_t* block = reinterpret_cast<buf_block_t*>(bpage);
@@ -1182,6 +1186,10 @@ static bool buf_LRU_block_remove_hashed(buf_page_t *bpage, const page_id_t id,
 		MEM_UNDEFINED(((buf_block_t*) bpage)->frame, srv_page_size);
 		bpage->set_state(BUF_BLOCK_REMOVE_HASH);
 
+		if (!zip) {
+			return true;
+		}
+
 		/* Question: If we release hash_lock here
 		then what protects us against:
 		1) Some other thread buffer fixing this page
@@ -1203,7 +1211,7 @@ static bool buf_LRU_block_remove_hashed(buf_page_t *bpage, const page_id_t id,
 		page_hash. */
 		hash_lock->write_unlock();
 
-		if (zip && bpage->zip.data) {
+		if (bpage->zip.data) {
 			/* Free the compressed page. */
 			void*	data = bpage->zip.data;
 			bpage->zip.data = NULL;