BUG #13893: PANIC: stuck spinlock

The following bug has been logged on the website:

Bug reference: 13893
Logged by: Chris Bainbridge
Email address: chris(dot)bainbridge(at)gmail(dot)com
PostgreSQL version: 9.4.5
Operating system: Debian Jessie
Description:

Sometimes on system shutdown postgresql hangs and the log shows:

PANIC: stuck spinlock (0x7f6089f088e0) detected at
/build/postgresql-9.4-CPUoxi/postgresql-9.4-9.4.5/build/../src/backend/storage/buffer/bufmgr.c:1292

or

2016-01-27 19:17:37 GMT [3351-1] PANIC: stuck spinlock (0x7f1de51dc8e0)
detected at
/build/postgresql-9.4-CPUoxi/postgresql-9.4-9.4.5/build/../src/backend/storage/buffer/bufmgr.c:1705

Which causes systemd to hang:

Jan 27 19:16:33 debian postgresql(at)9(dot)4-main[4959]: pg_ctl: server does not
shut down
Jan 27 19:16:33 debian systemd[1]: postgresql(at)9(dot)4-main(dot)service: control
process exited, code=exited status=1

Jan 27 19:17:37 debian systemd[1]: postgresql(at)9(dot)4-main(dot)service: main process
exited, code=exited, status=1/FAILURE
Jan 27 19:17:37 debian systemd[1]: Stopped PostgreSQL Cluster 9.4-main.
Jan 27 19:17:37 debian systemd[1]: Unit postgresql(at)9(dot)4-main(dot)service entered
failed state.
Jan 27 19:17:37 debian systemd[1]: Stopping system-postgresql.slice.

Sometimes the shutdown continues after a timeout, other times it seems to
hang indefinitely, sysrq-l shows postgres process as
http://imgur.com/kRAtvxw (Linux 4.4 kernel)

A similar bug was reported by another user a few months ago also for 9.4:
http://www.postgresql.org/message-id/20150810204319.2695.22673@wrigleys.postgresql.org