MDEV-34458 wait_for_read in buf_page_get_low hurts performance

BTR_MODIFY_PREV: Remove. This mode was only used by the change buffer,
which commit f27e9c89 (MDEV-29694)
removed.

buf_page_get_gen(): Revert the change that was made in
commit 90b95c61 (MDEV-33543)
because it is not applicable after MDEV-29694. This fixes the
performance regression that Vladislav Vaintroub reported.

This is a 11.x specific fix; this needs to be fixed differently
in older major versions where the change buffer is present.

storage/innobase/btr/btr0cur.cc

@@ -1220,7 +1220,7 @@ dberr_t btr_cur_t::search_leaf(const dtuple_t *tuple, page_cur_mode_t mode,
           ut_ad(rw_lock_type_t(latch_mode & ~12) == RW_X_LATCH);
           goto relatch_x;
         }
-        if (latch_mode != BTR_MODIFY_PREV)
+        else
         {
           if (!latch_by_caller)
             /* Release the tree s-latch */
@@ -1292,8 +1292,6 @@ dberr_t btr_cur_t::search_leaf(const dtuple_t *tuple, page_cur_mode_t mode,
 
     switch (latch_mode) {
     case BTR_SEARCH_PREV:
-    case BTR_MODIFY_PREV:
-      static_assert(BTR_MODIFY_PREV & BTR_MODIFY_LEAF, "");
       static_assert(BTR_SEARCH_PREV & BTR_SEARCH_LEAF, "");
       ut_ad(!latch_by_caller);
       ut_ad(rw_latch ==
@@ -1470,7 +1468,6 @@ dberr_t btr_cur_t::search_leaf(const dtuple_t *tuple, page_cur_mode_t mode,
     case BTR_MODIFY_ROOT_AND_LEAF:
       rw_latch= RW_X_LATCH;
       break;
-    case BTR_MODIFY_PREV: /* btr_pcur_move_to_prev() */
     case BTR_SEARCH_PREV: /* btr_pcur_move_to_prev() */
       ut_ad(rw_latch == RW_S_LATCH || rw_latch == RW_X_LATCH);
 
@@ -1854,7 +1851,6 @@ dberr_t btr_cur_t::open_leaf(bool first, dict_index_t *index,
     ut_ad(!(latch_mode & 8));
     /* This function doesn't need to lock left page of the leaf page */
     static_assert(int{BTR_SEARCH_PREV} == (4 | BTR_SEARCH_LEAF), "");
-    static_assert(int{BTR_MODIFY_PREV} == (4 | BTR_MODIFY_LEAF), "");
     latch_mode= btr_latch_mode(latch_mode & (RW_S_LATCH | RW_X_LATCH));
     ut_ad(!latch_by_caller ||
           mtr->memo_contains_flagged(&index->lock,

storage/innobase/btr/btr0pcur.cc

@@ -215,24 +215,18 @@ static bool btr_pcur_optimistic_latch_leaves(btr_pcur_t *pcur,
                                              btr_latch_mode *latch_mode,
                                              mtr_t *mtr)
 {
-  static_assert(BTR_SEARCH_PREV & BTR_SEARCH_LEAF, "");
-  static_assert(BTR_MODIFY_PREV & BTR_MODIFY_LEAF, "");
-  static_assert((BTR_SEARCH_PREV ^ BTR_MODIFY_PREV) ==
-                (RW_S_LATCH ^ RW_X_LATCH), "");
-
   buf_block_t *const block=
     buf_page_optimistic_fix(pcur->btr_cur.page_cur.block, pcur->old_page_id);
 
   if (!block)
     return false;
 
-  if (*latch_mode == BTR_SEARCH_LEAF || *latch_mode == BTR_MODIFY_LEAF)
+  if (*latch_mode != BTR_SEARCH_PREV)
+  {
+    ut_ad(*latch_mode == BTR_SEARCH_LEAF || *latch_mode == BTR_MODIFY_LEAF);
     return buf_page_optimistic_get(block, rw_lock_type_t(*latch_mode),
                                    pcur->modify_clock, mtr);
-
-  ut_ad(*latch_mode == BTR_SEARCH_PREV || *latch_mode == BTR_MODIFY_PREV);
-  const rw_lock_type_t mode=
-    rw_lock_type_t(*latch_mode & (RW_X_LATCH | RW_S_LATCH));
+  }
 
   uint64_t modify_clock;
   uint32_t left_page_no;
@@ -258,7 +252,7 @@ static bool btr_pcur_optimistic_latch_leaves(btr_pcur_t *pcur,
   {
     prev= buf_page_get_gen(page_id_t(pcur->old_page_id.space(),
                                      left_page_no), block->zip_size(),
-                           mode, nullptr, BUF_GET_POSSIBLY_FREED, mtr);
+                           RW_S_LATCH, nullptr, BUF_GET_POSSIBLY_FREED, mtr);
     if (!prev ||
         page_is_comp(prev->page.frame) != page_is_comp(block->page.frame) ||
         memcmp_aligned<2>(block->page.frame, prev->page.frame, 2) ||
@@ -269,7 +263,7 @@ static bool btr_pcur_optimistic_latch_leaves(btr_pcur_t *pcur,
   else
     prev= nullptr;
 
-  mtr->upgrade_buffer_fix(savepoint, mode);
+  mtr->upgrade_buffer_fix(savepoint, RW_S_LATCH);
 
   if (UNIV_UNLIKELY(block->modify_clock != modify_clock) ||
       UNIV_UNLIKELY(block->page.is_freed()) ||
@@ -343,11 +337,9 @@ btr_pcur_t::restore_position(btr_latch_mode restore_latch_mode, mtr_t *mtr)
 	ut_a(old_n_fields);
 
 	static_assert(BTR_SEARCH_PREV == (4 | BTR_SEARCH_LEAF), "");
-	static_assert(BTR_MODIFY_PREV == (4 | BTR_MODIFY_LEAF), "");
 
 	switch (restore_latch_mode | 4) {
 	case BTR_SEARCH_PREV:
-	case BTR_MODIFY_PREV:
 		/* Try optimistic restoration. */
 		if (btr_pcur_optimistic_latch_leaves(this, &restore_latch_mode,
 						     mtr)) {
@@ -579,7 +571,6 @@ btr_pcur_move_backward_from_page(
 	mtr_start(mtr);
 
 	static_assert(BTR_SEARCH_PREV == (4 | BTR_SEARCH_LEAF), "");
-	static_assert(BTR_MODIFY_PREV == (4 | BTR_MODIFY_LEAF), "");
 
 	if (UNIV_UNLIKELY(cursor->restore_position(
 				  btr_latch_mode(4 | latch_mode), mtr)

storage/innobase/buf/buf0buf.cc

@@ -2740,15 +2740,7 @@ buf_page_get_gen(
 		in buf_page_t::read_complete() or
 		buf_pool_t::corrupted_evict(), or
 		after buf_zip_decompress() in this function. */
-		if (rw_latch != RW_NO_LATCH) {
 		block->page.lock.s_lock();
-		} else if (!block->page.lock.s_lock_try()) {
-			/* For RW_NO_LATCH, we should not try to acquire S or X
-			latch directly as we could be violating the latching
-			order resulting in deadlock. Instead we try latching the
-			page and retry in case of a failure. */
-			goto wait_for_read;
-		}
 		state = block->page.state();
 		ut_ad(state < buf_page_t::READ_FIX
 		      || state >= buf_page_t::WRITE_FIX);
@@ -2756,15 +2748,15 @@ buf_page_get_gen(
 		block->page.lock.s_unlock();
 
 		if (UNIV_UNLIKELY(state < buf_page_t::UNFIXED)) {
+			block->page.unfix();
 			if (UNIV_UNLIKELY(id == page_id)) {
 				/* The page read was completed, and
 				another thread marked the page as free
 				while we were waiting. */
-				goto ignore_block;
+				goto ignore_unfixed;
 			}
 
 			ut_ad(id == page_id_t{~0ULL});
-			block->page.unfix();
 
 			if (++retries < BUF_PAGE_READ_MAX_RETRIES) {
 				goto loop;
@@ -2803,7 +2795,6 @@ buf_page_get_gen(
 	if (UNIV_UNLIKELY(!block->page.frame)) {
 		if (!block->page.lock.x_lock_try()) {
 wait_for_unzip:
-wait_for_read:
 			/* The page is being read or written, or
 			another thread is executing buf_zip_decompress()
 			in buf_page_get_gen() on it. */

storage/innobase/gis/gis0sea.cc

@@ -604,7 +604,6 @@ dberr_t rtr_search_to_nth_level(btr_cur_t *cur, que_thr_t *thr,
     upper_rw_latch= RW_X_LATCH;
     break;
   default:
-    ut_ad(latch_mode != BTR_MODIFY_PREV);
     ut_ad(latch_mode != BTR_SEARCH_PREV);
     if (!latch_by_caller)
       mtr_s_lock_index(index, mtr);

storage/innobase/include/btr0types.h

@@ -68,9 +68,6 @@ enum btr_latch_mode {
 	/** Search the previous record.
 	Used in btr_pcur_move_backward_from_page(). */
 	BTR_SEARCH_PREV = 4 | BTR_SEARCH_LEAF,
-	/** Modify the previous record.
-	Used in btr_pcur_move_backward_from_page(). */
-	BTR_MODIFY_PREV = 4 | BTR_MODIFY_LEAF,
 	/** Start modifying the entire B-tree. */
 	BTR_MODIFY_TREE = 8 | BTR_MODIFY_LEAF,
 	/** Continue modifying the entire R-tree.