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Per recent analysis by Andres Freund, this implementation is in fact unsafe, because ARMv5 has weak memory ordering, which means tha the CPU could move loads or stores across the volatile store performed by the default S_UNLOCK. We could try to fix this, but have no ARMv5 hardware to test on, so removing support seems better. We can still support ARMv5 systems on GCC versions new enough to have built-in atomics support for this platform, and can also re-add support for the old way if someone has hardware that can be used to test a fix. However, since the requirement to use a relatively-new GCC hasn't been an issue for ARMv6 or ARMv7, which lack the swpb instruction altogether, perhaps it won't be an issue for ARMv5 either.
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: http://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 http://www.postgresql.org/download/. For more information look at our web site located at http://www.postgresql.org/.