BUG #13900: stop standby failed with writer process hang(happen 3 times in 2 days)

The following bug has been logged on the website:

Bug reference: 13900
Logged by: Jov
Email address: amutu(at)amutu(dot)com
PostgreSQL version: 9.3.7
Operating system: FreeBSD 10.2 amd64
Description:

I am updating my 3 database from pg9.3 to pg9.5,but may find a bug for the
bgwriter of pg9.3.I can't stop all the stand by process,even for immediate
stop mode and kill -9,the writer process still there,with ps state "Ds" (D
Marks a process in disk (or other short term, uninterruptible) wait) .google
say the only method to clean the "Ds" process is rebooting the system.
truss say no info for the process,and procstat say the process is calling
the poll system call in the kernel.

These is the detail info:
pg_ctl -D ./slave stop -m fast
waiting for server to shut
down............................................................... failed
pg_ctl: server does not shut down

psql postgres
psql: FATAL: the database system is shutting down

pg_ctl -D ./slave stop -m immediate
waiting for server to shut down.... done
server stopped

ps auxwww | grep postgres
jovz 976 0.0 0.3 28840 5232 - Is 17 116 0:00.04 postgres:
logger process (postgres)
jovz 979 0.0 0.7 196940 13552 - Ds 17 116 0:06.03 postgres:
writer process (postgres)

log:
2016-01-30 14:23:22.350 CST,,,947,,569b1bc2.3b3,3,,2016-01-17 12:42:42
CST,,0,LOG,00000,"received fast shutdown request",,,,,,,,,""
2016-01-30 14:23:22.350 CST,,,947,,569b1bc2.3b3,4,,2016-01-17 12:42:42
CST,,0,LOG,00000,"aborting any active transactions",,,,,,,,,""
2016-01-30 14:25:35.271 CST,,,64815,"",56ac575f.fd2f,1,"",2016-01-30
14:25:35 CST,,0,LOG,00000,"connection received: host=[local]",,,,,,,,,""
2016-01-30 14:25:35.274
CST,"jovz","f",64815,"[local]",56ac575f.fd2f,2,"",2016-01-30 14:25:35
CST,,0,FATAL,57P03,"the database system is shutting down",,,,,,,,,""
2016-01-30 14:25:38.324 CST,,,64817,"",56ac5762.fd31,1,"",2016-01-30
14:25:38 CST,,0,LOG,00000,"connection received: host=[local]",,,,,,,,,""
2016-01-30 14:25:38.324
CST,"jovz","f",64817,"[local]",56ac5762.fd31,2,"",2016-01-30 14:25:38
CST,,0,FATAL,57P03,"the database system is shutting down",,,,,,,,,""
2016-01-30 14:47:36.727 CST,,,65457,"",56ac5c88.ffb1,1,"",2016-01-30
14:47:36 CST,,0,LOG,00000,"connection received: host=[local]",,,,,,,,,""
2016-01-30 14:47:36.727
CST,"jovz","postgres",65457,"[local]",56ac5c88.ffb1,2,"",2016-01-30 14:47:36
CST,,0,FATAL,57P03,"the database system is shutting down",,,,,,,,,""
2016-01-30 14:50:04.564 CST,,,947,,569b1bc2.3b3,5,,2016-01-17 12:42:42
CST,,0,LOG,00000,"received immediate shutdown request",,,,,,,,,""

truss -p 979

^Ctruss: Unexpect stop in waitpid: Interrupted system call
root(at)fblax:~ # procstat -kk 979
PID TID COMM TDNAME KSTACK

979 100688 postgres - mi_switch+0xe1
sleepq_timedwait_sig+0x8b _cv_timedwait_sig_sbt+0x18b seltdwait+0xa4
kern_poll+0x464 sys_poll+0x61 amd64_syscall+0x357 Xfast_syscall+0xfb
root(at)fb:~ # kill -9 979
root(at)fb:~ # procstat -kk 979
PID TID COMM TDNAME KSTACK

979 100688 postgres - mi_switch+0xe1
sleepq_timedwait_sig+0x8b _cv_timedwait_sig_sbt+0x18b seltdwait+0xa4
kern_poll+0x464 sys_poll+0x61 amd64_syscall+0x357 Xfast_syscall+0xfb