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This commit fixes two bugs in ProcKill()'s lock-group teardown freelist publication: * a double push of the leader's PGPROC that corrupts the freelist. * a leak of the last follower's PGPROC slot. ProcKill()'s lock-group teardown had two PGPROC freelist updates scattered through the function, done under two separate freeProcsLock acquisitions: * A follower's push of the leader's PGPROC, done when a follower is the last group member exiting. * Every backend's self-push at the bottom of the function. The two freelist updates were coordinated only by inspecting proc->lockGroupLeader, which a follower could clear as a side effect of pushing the leader. This coordination was broken. For example, with two concurrent backends: * The follower clears leader->lockGroupLeader and pushes the leader's PGPROC under leader_lwlock. * The follower does not clear its own proc->lockGroupLeader, being skipped. * When the leader reaches the bottom of ProcKill(), it sees a NULL proc->lockGroupLeader (the follower cleared it) and pushes itself, causing a second dlist_push_tail() of the same node onto the same freelist. * The follower at the bottom sees its own proc->lockGroupLeader being not NULL (never cleared) and skips its own push, causing its own slot to leak. This commit refactors the freelist manipulation to be done in two distinct phases, each step using its own lock acquisition to ensure that each freelist operation happens in an isolated manner for each backend (follower or leader): - First, under a single leader_lwlock acquisition, check the state of the lock-group. Depending on if we are dealing with a follower and/or a leader, and if the leader has exited before a follower, then set some state booleans that define which actions should be taken with the freelist. - Second, under a single freeProcsLock acquisition, perform the cleanup actions, self-push of a backend and/or push of the leader back to the freelist. This is an old issue, dating back to 9.6 where parallel workers and lock grouping has been added. Author: Vlad Lesin <vladlesin@gmail.com> Reviewed-by: Andrey Borodin <x4mmm@yandex-team.ru> Reviewed-by: Michael Paquier <michael@paquier.xyz> Discussion: https://postgr.es/m/d2983796-2603-41b7-a66e-fc8489ddb954@gmail.com Backpatch-through: 14 |
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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/.