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The lwlock scalability work introduced two race conditions into the lwlock variable support provided for xlog.c. First, and harmlessly on most platforms, it set/read the variable without the spinlock in some places. Secondly, due to the removal of the spinlock, it was possible that a backend missed changes to the variable's state if it changed in the wrong moment because checking the lock's state, the variable's state and the queuing are not protected by a single spinlock acquisition anymore. To fix first move resetting the variable's from LWLockAcquireWithVar to WALInsertLockRelease, via a new function LWLockReleaseClearVar. That prevents issues around waiting for a variable's value to change when a new locker has acquired the lock, but not yet set the value. Secondly re-check that the variable hasn't changed after enqueing, that prevents the issue that the lock has been released and already re-acquired by the time the woken up backend checks for the lock's state. Reported-By: Jeff Janes Analyzed-By: Heikki Linnakangas Reviewed-By: Heikki Linnakangas Discussion: 5592DB35.2060401@iki.fi Backpatch: 9.5, where the lwlock scalability went in |
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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/.