buf0block_hint.cc

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#include "buf0block_hint.h"
#include "buf0buf.h"
namespace buf {

void Block_hint::store(buf_block_t *block) {
  ut_ad(block->page.buf_fix_count > 0);
  m_block = block;
  m_page_id.copy_from(block->page.id);
}

void Block_hint::clear() { m_block = NULL; }

void Block_hint::buffer_fix_block_if_still_valid() {
  /* We need to check if m_block points to one of chunks. For this to be
  meaningful we need to prevent freeing memory while we check, and until we
  buffer-fix the block. For this purpose it is enough to latch any of the many
  latches taken by buf_resize().
  However, for buffer-fixing to be meaningful, the block has to contain a page
  (as opposed to being already empty, which might mean that buf_pool_resize()
  can proceed and free it once we free the s-latch), so we confirm that the
  block contains a page. However, it is not sufficient to check that this is
  just any page, because just after we check it could get freed, unless we
  have a latch which prevents this. This is tricky because page_hash latches
  are sharded by page_id and we don't know the page_id until we look into the
  block. To solve this chicken-and-egg problem somewhat, we latch the shard
  for the m_page_id and compare block->page.id to it - so if is equal then we
  can be reasonably sure that we have the correct latch.
  There is still a theoretical problem here, where other threads might try
  to modify the m_block->page.id while we are comparing it, but the chance of
  accidentally causing the old space_id == m_page_id.m_space and the new
  page_no == m_page_id.m_page_no is minimal as compilers emit a single 8-byte
  comparison instruction to compare both at the same time atomically, and f()
  will probably double-check the block->page.id again, anyway.
  Finally, assuming that we have correct hash bucket latched, we should check if
  the state of the block is BUF_BLOCK_FILE_PAGE before buffer-fixing the block,
  as otherwise we risk buffer-fixing and operating on a block, which is already
  meant to be freed. In particular, buf_LRU_free_page() first calls
  buf_LRU_block_remove_hashed() under hash bucket latch protection to change the
  state to BUF_BLOCK_REMOVE_HASH and then releases the latch. Later it calls
  buf_LRU_block_free_hashed_page() without any latch to change the state to
  BUF_BLOCK_MEMORY and reset the page's id, which means buf_resize() can free it
  regardless of our buffer-fixing. */
  if (m_block != NULL) {
    const buf_pool_t *const pool = buf_pool_get(m_page_id);
    rw_lock_t *latch = buf_page_hash_lock_get(pool, m_page_id);
    rw_lock_s_lock(latch);
    /* If not own buf_pool_mutex, page_hash can be changed. */
    latch = buf_page_hash_lock_s_confirm(latch, pool, m_page_id);
    if (buf_pointer_is_block_field_instance(pool, m_block) &&
        m_page_id.equals_to(m_block->page.id) &&
        buf_block_get_state(m_block) == BUF_BLOCK_FILE_PAGE) {
      buf_block_buf_fix_inc(m_block, __FILE__, __LINE__);
    } else {
      clear();
    }
    rw_lock_s_unlock(latch);
  }
}
void Block_hint::buffer_unfix_block_if_needed(buf_block_t *block) {
  if (block != NULL) {
    buf_block_buf_fix_dec(block);
  }
}
}  // namespace buf