[-next] system hangs likely due to "modules: Only return -EEXIST for modules that have finished loading"

[Heiko Carstens]

Hello Prarit,

it looks like your commit f9a75c1d717f ("modules: Only return -EEXIST
for modules that have finished loading") _sometimes_ causes hangs on
s390. This is unfortunately not 100% reproducible, however the
mentioned commit seems to be the only relevant one in modules.c.

What I see is a hanging system with messages like this on the console:

[   65.876040] rcu: INFO: rcu_sched self-detected stall on CPU
[   65.876049] rcu:     7-....: (5999 ticks this GP) idle=eae/1/0x4000000000000002 softirq=1181/1181 fqs=2729
[   65.876078]  (t=6000 jiffies g=-471 q=17196)
[   65.876084] Task dump for CPU 7:
[   65.876088] systemd-udevd   R  running task        0   731    721 0x06000004
[   65.876097] Call Trace:
[   65.876113] ([<0000000000abb264>] __schedule+0x2e4/0x6e0)
[   65.876122]  [<00000000001ee486>] finished_loading+0x4e/0xb0
[   65.876128]  [<00000000001f1ed6>] load_module+0xcce/0x27a0
[   65.876134]  [<00000000001f3af0>] __s390x_sys_init_module+0x148/0x178
[   65.876142]  [<0000000000ac0766>] system_call+0x2aa/0x2c8

crash's backtrace of this task looks like this:

PID: 731    TASK: 1e79ba000         CPU: 7   COMMAND: "systemd-udevd"
 LOWCORE INFO:
  -psw      : 0x0704c00180000000 0x0000000000ab666a
  -function : memcmp at ab666a
  -prefix   : 0x7fe32000
  -cpu timer: 0x7fffff5784d0f048
  -clock cmp: 0xd60757081cd70d00
  -general registers:
     0x0000000000000009 0x0000000000000009
     0x000003ff80347321 000000000000000000
     0x000003ff8034f321 000000000000000000
     0x000000000000001e 0x000003ff8c592708
     0x000003e0047da900 0x000003ff8034f318
     0x0000000000000001 0x0000000000000009
     0x000003ff80347300 0x0000000000ad81b8
     0x00000000001ee062 0x000003e004357cb0
[...]
 #0 [3e004357cf0] load_module at 1f1eb0
 #1 [3e004357dd0] __se_sys_init_module at 1f3af0
 #2 [3e004357ea8] system_call at ac0766
 PSW:  0705000180000000 000003ff8c27ad3e (user space)
 GPRS: 000002aa280d2fb0 000003ff8c27ad3c 0000000000000080 00000000000035fd
       000003ff8c592708 000002aa280c8770 0000000000000000 000002aa280d7130
       0000000000020000 000002aa28915760 000003ff8c592708 000002aa280c00b0
       000003ff8c8fff80 000002aa280c00b0 000003ff8c58a456 000003ffe9cfde90

I did not look any further into the dump, however since the commit
touches exactly the code path which seems to be looping... ;)