From 3420851a2c2d2ac49b8ba53ccec5d02aa1e6a272 Mon Sep 17 00:00:00 2001 From: Tom Lane Date: Mon, 29 Jul 2019 18:49:04 -0400 Subject: [PATCH] Fix busted logic for parallel lock grouping in TopoSort(). A "break" statement erroneously left behind by commit a1c1af2a1 caused TopoSort to do the wrong thing if a lock's wait list contained multiple members of the same locking group. Because parallel workers don't normally need any locks not already taken by their leader, this is very hard --- maybe impossible --- to hit in production. Still, if it did happen, the queries involved in an otherwise-resolvable deadlock would block until canceled. In addition to removing the bogus "break", add an Assert showing that the conflicting uses of the beforeConstraints[] array (for both counts and flags) don't overlap, and add some commentary explaining why not; because it's not obvious without explanation, IMHO. Original report and patch from Rui Hai Jiang; additional assert and commentary by me. Back-patch to 9.6 where the bug came in. Discussion: https://postgr.es/m/CAEri+mLd3bpHLyW+a9pSe1y=aEkeuJpwBSwvo-+m4n7-ceRmXw@mail.gmail.com --- src/backend/storage/lmgr/deadlock.c | 8 +++++++- 1 file changed, 7 insertions(+), 1 deletion(-) diff --git a/src/backend/storage/lmgr/deadlock.c b/src/backend/storage/lmgr/deadlock.c index 990d48377d..c687496b04 100644 --- a/src/backend/storage/lmgr/deadlock.c +++ b/src/backend/storage/lmgr/deadlock.c @@ -922,6 +922,12 @@ TopoSort(LOCK *lock, * in the same lock group on the queue, set their number of * beforeConstraints to -1 to indicate that they should be emitted * with their groupmates rather than considered separately. + * + * In this loop and the similar one just below, it's critical that we + * consistently select the same representative member of any one lock + * group, so that all the constraints are associated with the same + * proc, and the -1's are only associated with not-representative + * members. We select the last one in the topoProcs array. */ proc = constraints[i].waiter; Assert(proc != NULL); @@ -940,7 +946,6 @@ TopoSort(LOCK *lock, Assert(beforeConstraints[j] <= 0); beforeConstraints[j] = -1; } - break; } } @@ -977,6 +982,7 @@ TopoSort(LOCK *lock, if (kk < 0) continue; + Assert(beforeConstraints[jj] >= 0); beforeConstraints[jj]++; /* waiter must come before */ /* add this constraint to list of after-constraints for blocker */ constraints[i].pred = jj; -- 2.40.0