Clarify some comments

storage/innobase/buf/buf0buf.cc

@@ -2,7 +2,7 @@
 
 Copyright (c) 1995, 2018, Oracle and/or its affiliates. All Rights Reserved.
 Copyright (c) 2008, Google Inc.
-Copyright (c) 2013, 2020, MariaDB Corporation.
+Copyright (c) 2013, 2021, MariaDB Corporation.
 
 Portions of this file contain modifications contributed and copyrighted by
 Google, Inc. Those modifications are gratefully acknowledged and are described
@@ -3436,13 +3436,13 @@ buf_page_optimistic_get(
 	return(TRUE);
 }
 
-/** Given a tablespace id and page number tries to get that page. If the
-page is not in the buffer pool it is not loaded and NULL is returned.
-Suitable for using when holding the lock_sys_t::mutex.
-@param[in]	page_id	page id
-@param[in]	mtr	mini-transaction
-@return pointer to a page or NULL */
-buf_block_t* buf_page_try_get(const page_id_t page_id, mtr_t *mtr)
+/** Try to U-latch a page.
+Suitable for using when holding the lock_sys latches (as it avoids deadlock).
+@param[in]	page_id	page identifier
+@param[in,out]	mtr	mini-transaction
+@return the block
+@retval nullptr if an U-latch cannot be granted immediately */
+buf_block_t *buf_page_try_get(const page_id_t page_id, mtr_t *mtr)
 {
   ut_ad(mtr);
   ut_ad(mtr->is_active());

storage/innobase/include/buf0buf.h

 /*****************************************************************************
 
 Copyright (c) 1995, 2016, Oracle and/or its affiliates. All Rights Reserved.
-Copyright (c) 2013, 2020, MariaDB Corporation.
+Copyright (c) 2013, 2021, MariaDB Corporation.
 
 This program is free software; you can redistribute it and/or modify it under
 the terms of the GNU General Public License as published by the Free Software
@@ -232,13 +232,13 @@ buf_page_optimistic_get(
 	ib_uint64_t	modify_clock,/*!< in: modify clock value */
 	mtr_t*		mtr);	/*!< in: mini-transaction */
 
-/** Given a tablespace id and page number tries to get that page. If the
-page is not in the buffer pool it is not loaded and NULL is returned.
-Suitable for using when holding the lock_sys_t::mutex.
-@param[in]	page_id	page id
-@param[in]	mtr	mini-transaction
-@return pointer to a page or NULL */
-buf_block_t* buf_page_try_get(const page_id_t page_id, mtr_t *mtr);
+/** Try to U-latch a page.
+Suitable for using when holding the lock_sys latches (as it avoids deadlock).
+@param[in]	page_id	page identifier
+@param[in,out]	mtr	mini-transaction
+@return the block
+@retval nullptr if an U-latch cannot be granted immediately */
+buf_block_t *buf_page_try_get(const page_id_t page_id, mtr_t *mtr);
 
 /** Get read access to a compressed page (usually of type
 FIL_PAGE_TYPE_ZBLOB or FIL_PAGE_TYPE_ZBLOB2).

storage/innobase/trx/trx0trx.cc

 /*****************************************************************************
 
 Copyright (c) 1996, 2016, Oracle and/or its affiliates. All Rights Reserved.
-Copyright (c) 2015, 2020, MariaDB Corporation.
+Copyright (c) 2015, 2021, MariaDB Corporation.
 
 This program is free software; you can redistribute it and/or modify it under
 the terms of the GNU General Public License as published by the Free Software
@@ -1323,10 +1323,8 @@ inline void trx_t::commit_in_memory(const mtr_t *mtr)
     ut_a(!is_recovered);
     ut_ad(!rsegs.m_redo.rseg);
 
-    /* Note: We are asserting without holding the lock mutex. But
-    that is OK because this transaction is not waiting and cannot
-    be rolled back and no new locks can (or should) be added
-    because it is flagged as a non-locking read-only transaction. */
+    /* Note: We do not have to hold any lock_sys latch here, because
+    this is a non-locking transaction. */
     ut_a(UT_LIST_GET_LEN(lock.trx_locks) == 0);
 
     /* This state change is not protected by any mutex, therefore