MDEV-10813: Update buf_page_t::buf_fix_count outside mutex

Since MySQL 5.6.16 (and MariaDB Server 10.0.11), changes of
buf_page_t::buf_fix_count are atomic memory operations if
PAGE_ATOMIC_REF_COUNT is defined. Since MySQL 5.7
(and MariaDB Server 10.2.2), the field is always updated
by atomic memory operations.

In a few occurrences, updates of the counter were unnecessarily
surrounded by an acquisition and release of the block mutex
(buf_block_t::mutex or buf_pool_t::zip_mutex). Remove these
unnecessary mutex operations.

storage/innobase/btr/btr0cur.cc

@@ -463,11 +463,7 @@ btr_cur_optimistic_latch_leaves(
 					    file, line, mtr)) {
 			if (btr_page_get_prev(buf_block_get_frame(block), mtr)
 			    == left_page_no) {
-				/* adjust buf_fix_count */
-				buf_page_mutex_enter(block);
 				buf_block_buf_fix_dec(block);
-				buf_page_mutex_exit(block);
-
 				*latch_mode = mode;
 				return(true);
 			} else {
@@ -483,10 +479,7 @@ btr_cur_optimistic_latch_leaves(
 		}
 unpin_failed:
 		/* unpin the block */
-		buf_page_mutex_enter(block);
 		buf_block_buf_fix_dec(block);
-		buf_page_mutex_exit(block);
-
 		return(false);
 
 	default:

storage/innobase/buf/buf0buf.cc

@@ -4915,10 +4915,7 @@ buf_page_optimistic_get(
 	}
 
 	if (!success) {
-		buf_page_mutex_enter(block);
 		buf_block_buf_fix_dec(block);
-		buf_page_mutex_exit(block);
-
 		return(FALSE);
 	}
 
@@ -4932,10 +4929,7 @@ buf_page_optimistic_get(
 			rw_lock_x_unlock(&block->lock);
 		}
 
-		buf_page_mutex_enter(block);
 		buf_block_buf_fix_dec(block);
-		buf_page_mutex_exit(block);
-
 		return(FALSE);
 	}
 
@@ -5040,10 +5034,7 @@ buf_page_get_known_nowait(
 	}
 
 	if (!success) {
-		buf_page_mutex_enter(block);
 		buf_block_buf_fix_dec(block);
-		buf_page_mutex_exit(block);
-
 		return(FALSE);
 	}
 
@@ -5137,10 +5128,7 @@ buf_page_try_get_func(
 	}
 
 	if (!success) {
-		buf_page_mutex_enter(block);
 		buf_block_buf_fix_dec(block);
-		buf_page_mutex_exit(block);
-
 		return(NULL);
 	}
 

storage/innobase/fsp/fsp0fsp.cc

@@ -1374,11 +1374,8 @@ fsp_page_create(
 	} else {
 		rw_lock_sx_lock(&block->lock);
 	}
-	mutex_enter(&block->mutex);
 
 	buf_block_buf_fix_inc(block, __FILE__, __LINE__);
-
-	mutex_exit(&block->mutex);
 	mtr_memo_push(init_mtr, block, rw_latch == RW_X_LATCH
 		      ? MTR_MEMO_PAGE_X_FIX : MTR_MEMO_PAGE_SX_FIX);