66d86d4201
This commit adds a few comments about what LWLockWaitForVar() relies on when a backend waits for a variable update on its LWLocks for WAL insertions up to an expected LSN. First, LWLockWaitForVar() does not include a memory barrier, relying on a spinlock taken at the beginning of WaitXLogInsertionsToFinish(). This was hidden behind two layers of routines in lwlock.c. This assumption is now documented at the top of LWLockWaitForVar(), and detailed at bit more within LWLockConflictsWithVar(). Second, document why WaitXLogInsertionsToFinish() does not include memory barriers, relying on a spinlock at its top, which is, per Andres' input, fine for two different reasons, both depending on the fact that the caller of WaitXLogInsertionsToFinish() is waiting for a LSN up to a certain value. This area's documentation and assumptions could be improved more in the future, but at least that's a beginning. Author: Bharath Rupireddy, Andres Freund Reviewed-by: Michael Paquier Discussion: https://postgr.es/m/CALj2ACVF+6jLvqKe6xhDzCCkr=rfd6upaGc3477Pji1Ke9G7Bg@mail.gmail.com
PostgreSQL Database Management System ===================================== This directory contains the source code distribution of the PostgreSQL database management system. PostgreSQL is an advanced object-relational database management system that supports an extended subset of the SQL standard, including transactions, foreign keys, subqueries, triggers, user-defined types and functions. This distribution also contains C language bindings. PostgreSQL has many language interfaces, many of which are listed here: https://www.postgresql.org/download/ See the file INSTALL for instructions on how to build and install PostgreSQL. That file also lists supported operating systems and hardware platforms and contains information regarding any other software packages that are required to build or run the PostgreSQL system. Copyright and license information can be found in the file COPYRIGHT. A comprehensive documentation set is included in this distribution; it can be read as described in the installation instructions. The latest version of this software may be obtained at https://www.postgresql.org/download/. For more information look at our web site located at https://www.postgresql.org/.