Fix race conditions in ProcKill()'s lock-group freelist handling 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 <[email protected]> Reviewed-by: Andrey Borodin <[email protected]> Reviewed-by: Michael Paquier <[email protected]> Discussion: https://postgr.es/m/[email protected] Backpatch-through: 14 Branch ------ master Details ------- https://git.postgresql.org/pg/commitdiff/56310452318cee4bb8c291be126ce0c2b2affa5f Modified Files -------------- src/backend/storage/lmgr/proc.c | 77 +++++++++++++++++++++++++++-------------- 1 file changed, 51 insertions(+), 26 deletions(-)
