On 4/26/19 2:10 PM, Prarit Bhargava wrote:
>
>
> On 4/26/19 12:09 PM, Jessica Yu wrote:
>> +++ Heiko Carstens [26/04/19 17:07 +0200]:
>>> On Fri, Apr 26, 2019 at 09:22:34AM -0400, Prarit Bhargava wrote:
>>>> On 4/26/19 9:07 AM, Heiko Carstens wrote:
>>>>> 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
>>>>> I did not look any further into the dump, however since the commit
>>>>> touches exactly the code path which seems to be looping... ;)
>>>>>
>>>>
>>>> Ouch :( I wonder if I exposed a further race or another bug. Heiko, can you
>>>> determine which module is stuck? Warning: I have not compiled this code.
>>>
>>> Here we go:
>>>
>>> [ 11.716866] PRARIT: waiting for module s390_trng to load.
>>> [ 11.716867] PRARIT: waiting for module s390_trng to load.
>>> [ 11.716868] PRARIT: waiting for module s390_trng to load.
>>> [ 11.716870] PRARIT: waiting for module s390_trng to load.
>>> [ 11.716871] PRARIT: waiting for module s390_trng to load.
>>> [ 11.716872] PRARIT: waiting for module s390_trng to load.
>>> [ 11.716874] PRARIT: waiting for module s390_trng to load.
>>> [ 11.716875] PRARIT: waiting for module s390_trng to load.
>>> [ 11.716876] PRARIT: waiting for module s390_trng to load.
>>> [ 16.726850] add_unformed_module: 31403529 callbacks suppressed
>>> [ 16.726853] PRARIT: waiting for module s390_trng to load.
>>> [ 16.726862] PRARIT: waiting for module s390_trng to load.
>>> [ 16.726865] PRARIT: waiting for module s390_trng to load.
>>> [ 16.726867] PRARIT: waiting for module s390_trng to load.
>>> [ 16.726869] PRARIT: waiting for module s390_trng to load.
>>>
>>> If I'm not mistaken then there was _no_ corresponding message on the
>>> console stating that the module already exists.
>
> Heiko,
>
> Where is the s390_trng module? I can't seem to find it in the tree.
>
> [02:06 PM root@ibm-z-108 linux-next]# find ./ -name *s390_trng*
> [02:06 PM root@ibm-z-108 linux-next]# git grep s390_trng
> [02:07 PM root@ibm-z-108 linux-next]#
>
Never mind, I found it: s390-trng.c
P.
> P.
>
>>
>> Hm, my current theory is that we have a module whose exit() function
>> is taking a while to run to completion. While it is doing so, the
>> module's state is already set to MODULE_STATE_GOING.
>>
>> With Prarit's patch, since this module is probably still in GOING,
>> add_unformed_module() will wait until the module is finally gone. If
>> this takes too long, we will keep trying to add ourselves to the
>> module list and hence stay in the loop in add_unformed_module().
>> According to Documentation/RCU/stallwarn.txt, this looping in the
>> kernel may trigger an rcu stall warning (see bullet point stating "a
>> CPU looping anywhere in the kernel without invoking schedule()".
>>
>> Heiko, could you modify the patch to print the module's state to
>> confirm?
>>
>> Thanks,
>>
>> Jessica
