Backends stuck in LISTEN

Hi people,

Today we had a problem where around 30-40 backends were stuck in LISTEN
for a long time. I snatched a core dump before we had to kill the
server, and the backtrace looks like this:

#0 0x00007f26dd17d5b7 in monstartup () from /lib/x86_64-linux-gnu/libc.so.6
#1 0x00007f26d79e5c70 in ?? ()
#2 0x00007f26d79e5c80 in ?? ()
#3 0x00007f254d772b60 in ?? ()
#4 0x00000000006359c5 in LWLockAcquire (lockid=87, mode=LW_SHARED) at
/build/buildd/postgresql-9.1-9.1.9/build/../src/backend/storage/lmgr/lwlock.c:464
#5 0x000000000049fabb in SimpleLruWaitIO (slotno=6, ctl=<optimized
out>) at
/build/buildd/postgresql-9.1-9.1.9/build/../src/backend/access/transam/slru.c:306
#6 0x000000000049fe00 in SlruSelectLRUPage (ctl=<optimized out>,
pageno=34331) at
/build/buildd/postgresql-9.1-9.1.9/build/../src/backend/access/transam/slru.c:994
#7 0x00000000004a02ab in SimpleLruReadPage (ctl=0xb046a0, pageno=34331,
write_ok=1 '\001', xid=1124964531) at
/build/buildd/postgresql-9.1-9.1.9/build/../src/backend/access/transam/slru.c:366
#8 0x0000000000487fb3 in TransactionIdGetStatus (xid=1124964531,
lsn=0x7fffe8503750) at
/build/buildd/postgresql-9.1-9.1.9/build/../src/backend/access/transam/clog.c:400
#9 0x0000000000488460 in TransactionLogFetch (transactionId=1124964531)
at
/build/buildd/postgresql-9.1-9.1.9/build/../src/backend/access/transam/transam.c:83
#10 TransactionLogFetch (transactionId=1124964531) at
/build/buildd/postgresql-9.1-9.1.9/build/../src/backend/access/transam/transam.c:56
#11 0x00000000004885b1 in TransactionIdDidCommit
(transactionId=<optimized out>) at
/build/buildd/postgresql-9.1-9.1.9/build/../src/backend/access/transam/transam.c:133
#12 0x00000000005159f4 in asyncQueueProcessPageEntries
(page_buffer=0x7fffe85038b0 "$", stop=..., current=<synthetic pointer>)
at
/build/buildd/postgresql-9.1-9.1.9/build/../src/backend/commands/async.c:1998
#13 asyncQueueReadAllNotifications () at
/build/buildd/postgresql-9.1-9.1.9/build/../src/backend/commands/async.c:1923
#14 0x0000000000516926 in Exec_ListenPreCommit () at
/build/buildd/postgresql-9.1-9.1.9/build/../src/backend/commands/async.c:977
#15 PreCommit_Notify () at
/build/buildd/postgresql-9.1-9.1.9/build/../src/backend/commands/async.c:801
#16 0x000000000048c863 in CommitTransaction () at
/build/buildd/postgresql-9.1-9.1.9/build/../src/backend/access/transam/xact.c:1827
#17 0x000000000048ce8d in CommitTransactionCommand () at
/build/buildd/postgresql-9.1-9.1.9/build/../src/backend/access/transam/xact.c:2563
#18 0x000000000063fafb in finish_xact_command () at
/build/buildd/postgresql-9.1-9.1.9/build/../src/backend/tcop/postgres.c:2452
#19 finish_xact_command () at
/build/buildd/postgresql-9.1-9.1.9/build/../src/backend/tcop/postgres.c:2441
#20 0x0000000000641e92 in PostgresMain (argc=<optimized out>,
argv=<optimized out>, dbname=0xc5dee0 "gluepay", username=<optimized
out>) at
/build/buildd/postgresql-9.1-9.1.9/build/../src/backend/tcop/postgres.c:4153
#21 0x0000000000603f83 in BackendRun (port=0xca6f50) at
/build/buildd/postgresql-9.1-9.1.9/build/../src/backend/postmaster/postmaster.c:3612
#22 BackendStartup (port=0xca6f50) at
/build/buildd/postgresql-9.1-9.1.9/build/../src/backend/postmaster/postmaster.c:3302
#23 ServerLoop () at
/build/buildd/postgresql-9.1-9.1.9/build/../src/backend/postmaster/postmaster.c:1466
#24 0x00000000006049f1 in PostmasterMain (argc=<optimized out>,
argv=<optimized out>) at
/build/buildd/postgresql-9.1-9.1.9/build/../src/backend/postmaster/postmaster.c:1127
#25 0x000000000045a600 in main (argc=5, argv=0xc5d180) at
/build/buildd/postgresql-9.1-9.1.9/build/../src/backend/main/main.c:199

This is on 64-bit Linux, version 9.1.9 from the apt repositories.

Any idea what could cause this? It took around 30 seconds for the
backends to respond to pg_terminate_backend().

Regards,
Marko Tiikkaja