Fix segfault from releasing locks in detached DSM segments If a FATAL error occurs while holding a lock in a DSM segment (such as a dshash lock) and the process is not in a transaction, a segmentation fault can occur during process exit.
The problem sequence is: 1. Process acquires a lock in a DSM segment (e.g., via dshash) 2. FATAL error occurs outside transaction context 3. proc_exit() begins, calling before_shmem_exit callbacks 4. dsm_backend_shutdown() detaches all DSM segments 5. Later, on_shmem_exit callbacks run 6. ProcKill() calls LWLockReleaseAll() 7. Segfault: the lock being released is in unmapped memory This only manifests outside transaction contexts because AbortTransaction() calls LWLockReleaseAll() during transaction abort, releasing locks before DSM cleanup. Background workers and other non-transactional code paths are vulnerable. Fix by calling LWLockReleaseAll() unconditionally at the start of shmem_exit(), before any callbacks run. Releasing locks before callbacks prevents the segfault - locks must be released before dsm_backend_shutdown() detaches their memory. This is safe because after an error, held locks are protecting potentially inconsistent data anyway, and callbacks can acquire fresh locks if needed. Also add a comment noting that LWLockReleaseAll() must be safe to call before LWLock initialization (which it is, since num_held_lwlocks will be 0), plus an Assert for the post-condition. This fix aligns with the original design intent from commit 001a573a2, which noted that backends must clean up shared memory state (including releasing lwlocks) before unmapping dynamic shared memory segments. Reported-by: Rahila Syed <[email protected]> Author: Rahila Syed <[email protected]> Reviewed-by: Amit Langote <[email protected]> Reviewed-by: Dilip Kumar <[email protected]> Reviewed-by: Andres Freund <[email protected]> Reviewed-by: Álvaro Herrera <[email protected]> Discussion: https://postgr.es/m/CAH2L28uSvyiosL+kaic9249jRVoQiQF6JOnaCitKFq=xifz...@mail.gmail.com Backpatch-through: 14 Branch ------ REL_16_STABLE Details ------- https://git.postgresql.org/pg/commitdiff/980b7c7369e44e49078ad2bbb39e7012347a0919 Modified Files -------------- src/backend/storage/ipc/ipc.c | 11 +++++++++-- src/backend/storage/lmgr/lwlock.c | 6 ++++++ 2 files changed, 15 insertions(+), 2 deletions(-)
