I noticed in /usr/lib/systemd/system/multipathd.service, ExecReload is '/sbin/multipathd reconfigure'.

The reload command completes successfully, is this a workaround?  Or failing on the backend where we can't see?

[root@localhost multipath-tools]# multipathd -k"reconfigure"
error receiving packet
[root@localhost multipath-tools]# multipathd reconfigure
error receiving packet
[root@localhost multipath-tools]# systemctl reload multipathd.service
[root@localhost multipath-tools]# echo $?
0

The issue seems to be with poll() timing out as we can see from a strace.  The timeout occurs with either multipathd -k reconfigured or /sbin/multipathd/reconfigure:

31183 16:23:56.966192 socket(PF_LOCAL, SOCK_STREAM, 0) = 3 <0.001124>
31183 16:23:56.967338 connect(3, {sa_family=AF_LOCAL, sun_path=@"/org/kernel/linux/storage/multipathd"}, 39) = 0 <0.000713>
31183 16:23:56.968079 rt_sigprocmask(SIG_BLOCK, [PIPE], [], 8) = 0 <0.000007>
31183 16:23:56.968112 write(3, "\16\0\0\0\0\0\0\0", 8) = 8 <0.000010>
31183 16:23:56.968136 write(3, "reconfigure \n\0", 14) = 14 <0.000008>
31183 16:23:56.968156 rt_sigprocmask(SIG_SETMASK, [], NULL, 8) = 0 <0.000007>
31183 16:23:56.968179 poll([{fd=3, events=POLLIN}], 1, 10000) = 0 (Timeout) <10.010081>

The threshold seems to be 10s: 

# time multipathd -kreconfigure
ok

real	0m9.512s
user	0m0.000s
sys	0m0.003s

# time multipathd -kreconfigure
error receiving packet

real	0m10.006s
user	0m0.001s
sys	0m0.003s

No Drops and the receive buffer seems large enough: 

sk       Eth Pid    Groups   Rmem     Wmem     Dump     Locks     Drops     Inode
ffff88ff23a6b000 15  3322   00000002 0        0        0 2        0        30769
UNCONN     0      0      uevent:multipathd/3322           *
         skmem:(r0,rb268435456,t0,tb212992,f0,w0,o96,bl0)
--
UNCONN     0      0      uevent:multipathd/3322           *
         skmem:(r0,rb268435456,t0,tb212992,f0,w0,o96,bl0)

Have not been able to reproduce this issue at F27 levels, the command returns much quicker.

device-mapper-multipath-0.7.1-9.git847cc43.fc27
device-mapper-1.02.144-1.fc27
kernel-4.14.18-300.fc27

$ multipath -ll | grep -c dm-
1003
$ time multipathd -k"reconfigure"
ok

real   0m0.088s
user   0m0.004s
sys    0m0.004s

f27 does reconfiguration completely differently. That command returns as soon as multipathd starts doing the reconfiguration, instead of when it finishes. Multipathd continues to use the old configuration until reconfiguration completes, and then it switches over. So this shouldn't be an issue in f27 and beyond. For rhel7, this is probably simply a matter of increasing the timeout. Reconfiguration will complete at any rate, the multipathd command client simply times out waiting for a reply from multipathd. I don't think this is exception or blocker worthy (which means it will miss rhel-7.5), since the reconfiguration does happen, and most users use systemctl to reconfigure multipathd anyway, and won't even notice an issue.

Thanks a lot Ben, that's great news that the socket error does not mean a failed command.  I saw that a uxsock_timeout keyword is implemented in later versions, 96e8b90227ce (Implement 'uxsock_timeout' keyword).  In theory, would this parameter, if set, have been a workaround for the error?

Yes. uxsock_timeout controls how long multipathd will wait for a message from a command client, and how long the client will wait for a reply from multipathd. So increasing that would have solved the issue.

Thanks again Ben.  Decreasing sev/prio.

I've increased the timeout from 10 seconds to 60 seconds.

Reproduced on device-mapper-multipath-0.4.9-111
1, [root@storageqe-06 ~]# rpm -qa | grep multipath
device-mapper-multipath-0.4.9-111.el7.x86_64
device-mapper-multipath-libs-0.4.9-111.el7.x86_64

2,[root@storageqe-06 ~]# modprobe scsi_debug virtual_gb=1 add_host=2 num_tgts=500
[root@storageqe-06 ~]# multipathd -k"reconfigure"
ok

3,[root@storageqe-06 ~]# modprobe -r scsi_debug
[root@storageqe-06 ~]# modprobe scsi_debug virtual_gb=1 add_host=2 num_tgts=600
[root@storageqe-06 ~]# time multipathd -k"reconfigure"
ok

real	0m2.549s
user	0m0.001s
sys	0m0.003s

4,[root@storageqe-06 ~]# modprobe -r scsi_debug
[root@storageqe-06 ~]# modprobe scsi_debug virtual_gb=1 add_host=2 num_tgts=700
[root@storageqe-06 ~]# time multipathd -k"reconfigure"
error receiving packet  <----------------------------

real	0m10.014s
user	0m0.002s
sys	0m0.002s


Verified on device-mapper-multipath-0.4.9-121.el7
1,[root@storageqe-06 ~]# rpm -qa | grep multipath 
device-mapper-multipath-0.4.9-121.el7.x86_64
device-mapper-multipath-libs-0.4.9-121.el7.x86_64
[root@storageqe-06 ~]# modprobe scsi_debug virtual_gb=1 add_host=2 num_tgts=700

2,[root@storageqe-06 ~]# time multipathd -k"reconfigure"
ok

real	0m13.626s
user	0m0.001s
sys	0m0.003s

3,[root@storageqe-06 ~]# modprobe -r scsi_debug
[root@storageqe-06 ~]# modprobe scsi_debug virtual_gb=1 add_host=2 num_tgts=800
[root@storageqe-06 ~]# time multipathd -k"reconfigure"
ok

real	0m0.130s
user	0m0.002s
sys	0m0.002s



Test result: Both attempts were successful over 10s threshold

Since the problem described in this bug report should be
resolved in a recent advisory, it has been closed with a
resolution of ERRATA.

For information on the advisory, and where to find the updated
files, follow the link below.

If the solution does not work for you, open a new bug report.

https://access.redhat.com/errata/RHBA-2018:3236