Greeting's

multiple task hung while adding the vfc disk back to the multipath on my 
powerpc box running linux-next kernel

Test:
$ multipathd -k"remove path sde"
$ multipathd -k"add path sde"

After above command multipathd task hung with continuous call traces on 
console, it requires reboot to stop call traces

systemd-udevd[180359]: Process '/sbin/mdadm -I /dev/dm-8' failed with 
exit code 1.
multipathd[180272]: mpathf: load table [0 62914560 multipath 1 
queue_if_no_path 1 alua 2 1 round-robin 0 2 1 8:64 1 65:0 1 round-robin 
0 2 1 8:144 1 65:80 1]
systemd[1]: Started Device-Mapper Multipath Device Controller.
multipathd[180272]: mpatha: sdl - tur checker reports path is up
multipathd[180272]: 8:176: reinstated
multipathd[180272]: mpatha: remaining active paths: 2
multipathd[180272]: sde: remove path (operator)
multipathd[180272]: mpathf: load table [0 62914560 multipath 1 
queue_if_no_path 1 alua 2 1 round-robin 0 1 1 65:0 1 round-robin 0 2 1 
8:144 1 65:80 1]
multipathd[180272]: sde [8:64]: path removed from map mpathf
multipathd[180272]: sdq: remove path (operator)
multipathd[180272]: mpathf: load table [0 62914560 multipath 1 
queue_if_no_path 1 alua 1 1 round-robin 0 2 1 8:144 1 65:80 1]
multipathd[180272]: sdq [65:0]: path removed from map mpathf
multipathd[180272]: sdj: remove path (operator)
multipathd[180272]: mpathf: load table [0 62914560 multipath 1 
queue_if_no_path 1 alua 1 1 round-robin 0 1 1 65:80 1]
multipathd[180272]: sdj [8:144]: path removed from map mpathf
multipathd[180272]: sdv: remove path (operator)
multipathd[180272]: mpathf: map in use
multipathd[180272]: mpathf: can't flush
multipathd[180272]: mpathf: load table [0 62914560 multipath 1 
queue_if_no_path 0 0 0]
multipathd[180272]: sdv [65:80]: path removed from map mpathf
systemd[1]: Starting system activity accounting tool...
systemd[1]: sysstat-collect.service: Succeeded.
systemd[1]: Started system activity accounting tool.
systemd-udevd[1156]: seq 5678 '/devices/virtual/block/dm-10' is taking a 
long time
systemd-udevd[1156]: seq 5678 '/devices/virtual/block/dm-10' killed
multipathd[180272]: sde: add path (operator)
systemd[1]: Stopping Device-Mapper Multipath Device Controller...
systemd[1]: multipathd.service: State 'stop-sigterm' timed out. Killing.
systemd[1]: multipathd.service: Killing process 180272 (multipathd) with 
signal SIGKILL.
systemd[1]: multipathd.service: Processes still around after SIGKILL. 
Ignoring.
dbus-daemon[1920]: [system] Activating via systemd: service 
name='net.reactivated.Fprint' unit='fprintd.service' requested by 
':1.255' (uid=0 pid=95173 comm="/bin/login -p --      ")
systemd[1]: Starting Fingerprint Authentication Daemon...
dbus-daemon[1920]: [system] Successfully activated service 
'net.reactivated.Fprint'
systemd[1]: Started Fingerprint Authentication Daemon.
systemd-logind[2032]: New session 6 of user root.
systemd[1]: Started Session 6 of user root.
kernel: INFO: task multipathd:180274 blocked for more than 122 seconds.
kernel:      Not tainted 5.14.0-rc7-next-20210827-autotest #1
kernel: "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this 
message.
kernel: task:multipathd      state:D stack:    0 pid:180274 ppid:     1 
flags:0x00040082
kernel: Call Trace:
kernel: [c00000002aaa72c0] [c00000002aaa73a0] 0xc00000002aaa73a0 
(unreliable)
kernel: [c00000002aaa74b0] [c00000000001e638] __switch_to+0x278/0x490
kernel: [c00000002aaa7510] [c000000000c89cfc] __schedule+0x31c/0xa10
kernel: [c00000002aaa75d0] [c000000000c8a468] schedule+0x78/0x130
kernel: [c00000002aaa7600] [c000000000c8aa08] 
schedule_preempt_disabled+0x18/0x30
kernel: [c00000002aaa7620] [c000000000c8cf0c] 
__mutex_lock.isra.11+0x36c/0x700
kernel: [c00000002aaa76b0] [c00000000067efe4] bd_link_disk_holder+0x34/0x270
kernel: [c00000002aaa7700] [c0080000003e3b78] 
dm_get_table_device+0x1e0/0x2c0 [dm_mod]
kernel: [c00000002aaa77a0] [c0080000003e7e48] dm_get_device+0x130/0x2f0 
[dm_mod]
kernel: [c00000002aaa7850] [c008000000745234] multipath_ctr+0x9bc/0xff0 
[dm_multipath]
kernel: [c00000002aaa79d0] [c0080000003e883c] 
dm_table_add_target+0x1a4/0x420 [dm_mod]
kernel: [c00000002aaa7a90] [c0080000003ee874] table_load+0x15c/0x4a0 
[dm_mod]
kernel: [c00000002aaa7b40] [c0080000003f1454] ctl_ioctl+0x27c/0x770 [dm_mod]
kernel: [c00000002aaa7d40] [c0080000003f1960] dm_ctl_ioctl+0x18/0x30 
[dm_mod]
kernel: [c00000002aaa7d60] [c000000000481198] sys_ioctl+0xf8/0x150
kernel: [c00000002aaa7db0] [c00000000002ff28] 
system_call_exception+0x158/0x2c0
kernel: [c00000002aaa7e10] [c00000000000c764] system_call_common+0xf4/0x258
kernel: --- interrupt: c00 at 0x7fffa06f4480
kernel: NIP:  00007fffa06f4480 LR: 00007fffa0ad6714 CTR: 0000000000000000
kernel: REGS: c00000002aaa7e80 TRAP: 0c00   Not tainted 
(5.14.0-rc7-next-20210827-autotest)
kernel: MSR:  800000000000d033 <SF,EE,PR,ME,IR,DR,RI,LE>  CR: 24042204  
XER: 00000000
kernel: IRQMASK: 0 #012GPR00: 0000000000000036 00007fff9fd4c3a0 
00007fffa07e7100 0000000000000005 #012GPR04: 00000000c138fd09 
00007fff9806a0c0 00007fffa0ad9f18 00007fff9fd4a298 #012GPR08: 
0000000000000005 0000000000000000 0000000000000000 0000000000000000 
#012GPR12: 0000000000000000 00007fff9fd56300 00007fff9806a0c0 
00007fffa0ad9c80 #012GPR16: 00007fffa0ad9c80 00007fffa0ad9c80 
00007fffa0b13670 0000000000000000 #012GPR20: 00007fffa0ae3260 
00007fffa0b12040 00007fff9806a0f0 00007fff9800eca0 #012GPR24: 
00007fffa0ad9c80 00007fffa0ad9c80 00007fffa0ad9c80 0000000000000000 
#012GPR28: 00007fffa0ad9c80 00007fffa0ad9c80 0000000000000000 
00007fffa0ad9c80
kernel: NIP [00007fffa06f4480] 0x7fffa06f4480
kernel: LR [00007fffa0ad6714] 0x7fffa0ad6714
kernel: --- interrupt: c00
kernel: INFO: task systemd-udevd:180404 blocked for more than 122 seconds.
kernel:      Not tainted 5.14.0-rc7-next-20210827-autotest #1
kernel: "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this 
message.
kernel: task:systemd-udevd   state:D stack:    0 pid:180404 ppid:  1156 
flags:0x00042482
kernel: Call Trace:

Before test
------------
$multipath -ll
mpathf (360050768108001b3a8000000000000e8) dm-10 IBM,2145
size=30G features='1 queue_if_no_path' hwhandler='1 alua' wp=rw
|-+- policy='round-robin 0' prio=50 status=active
| |- 1:0:0:4 sde 8:64  active ready running
| `- 2:0:0:4 sdq 65:0  active ready running
`-+- policy='round-robin 0' prio=10 status=enabled
   |- 1:0:1:4 sdj 8:144 active ready running
   `- 2:0:1:4 sdv 65:80 active ready running
$

After test fail
------------
$ multipath -ll
mpathf (360050768108001b3a8000000000000e8) dm-10 ##,##
size=30G features='1 queue_if_no_path' hwhandler='0' wp=rw
$

-- 
Regard's

Abdul Haleem
IBM Linux Technology Center