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In LWLockRelease() (and in 9.4+ LWLockUpdateVar()) we release enqueued waiters using PGSemaphoreUnlock(). As there are other sources of such unlocks backends only wake up if MyProc->lwWaiting is set to false; which is only done in the aforementioned functions. Before this commit there were dangers because the store to lwWaitLink could become visible before the store to lwWaitLink. This could both happen due to compiler reordering (on most compilers) and on some platforms due to the CPU reordering stores. The possible consequence of this is that a backend stops waiting before lwWaitLink is set to NULL. If that backend then tries to acquire another lock and has to wait there the list could become corrupted once the lwWaitLink store is finally performed. Add a write memory barrier to prevent that issue. Unfortunately the barrier support has been only added in 9.2. Given that the issue has not knowingly been observed in praxis it seems sufficient to prohibit compiler reordering using volatile for 9.0 and 9.1. Actual problems due to compiler reordering are more likely anyway. Discussion: 20140210134625.GA15246@awork2.anarazel.de |
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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: 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/.