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In the generic atomic ops that rely on a loop around a CAS primitive, there's no need to force the initial read of the "old" value to be atomic. In the typically-rare case that we get a torn value, that simply means that the first CAS attempt will fail; but it will update "old" to the atomically-read value, so the next attempt has a chance of succeeding. It was already being done that way in pg_atomic_exchange_u64_impl(), but let's duplicate the approach in the rest. (Given the current coding of the pg_atomic_read functions, this change is a no-op anyway on popular platforms; it only makes a difference where pg_atomic_read_u64_impl() is implemented as a CAS.) In passing, also remove unnecessary take-a-pointer-and-dereference-it coding in the pg_atomic_read functions. That seems to have been based on a misunderstanding of what the C standard requires. What actually matters is that the pointer be declared as pointing to volatile, which it is. I don't believe this will change the assembly code at all on x86 platforms (even ignoring the likelihood that these implementations get overridden by others); but it may help on less-mainstream CPUs. Discussion: https://postgr.es/m/13707.1504718238@sss.pgh.pa.us |
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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/.