When two (or more) transactions are waiting for transaction T1 to release a tuple-level lock, and transaction T1 upgrades its lock to a higher level, a spurious deadlock can be reported among the waiting transactions when T1 finishes. The simplest example case seems to be:
T1: select id from job where name = 'a' for key share; Y: select id from job where name = 'a' for update; -- starts waiting for X Z: select id from job where name = 'a' for key share; T1: update job set name = 'b' where id = 1; Z: update job set name = 'c' where id = 1; -- starts waiting for X T1: rollback;
At this point, transaction Y is rolled back on account of a deadlock: Y holds the heavyweight tuple lock and is waiting for the Xmax to be released, while Z holds part of the multixact and tries to acquire the heavyweight lock (per protocol) and goes to sleep; once X releases its part of the multixact, Z is awakened only to be put back to sleep on the heavyweight lock that Y is holding while sleeping. Kaboom.
This can be avoided by having Z skip the heavyweight lock acquisition. As far as I can see, the biggest downside is that if there are multiple Z transactions, the order in which they resume after X finishes is not guaranteed.
Backpatch to 9.6. The patch applies cleanly on 9.5, but the new tests don't work there (because isolationtester is not smart enough), so I'm not going to risk it.
Author: Oleksii Kliukin Discussion: https://postgr.es/m/B9C9D7CD-EB94-4635-91B6-E558ACEC0EC3@hintbits.com
de87a084c0 Avoid spurious deadlocks when upgrading a tuple lock
src/backend/access/heap/README.tuplock | 10 ++
src/backend/access/heap/heapam.c | 84 +++++++++---
.../expected/tuplelock-upgrade-no-deadlock.out | 150 +++++++++++++++++++++
src/test/isolation/isolation_schedule | 1 +
.../specs/tuplelock-upgrade-no-deadlock.spec | 57 ++++++++
5 files changed, 281 insertions(+), 21 deletions(-)