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

From: Prarit Bhargava <[email-masked]>
Date: Sat, 27 Apr 2019 06:35:06 -0400

On 4/27/19 6:24 AM, Heiko Carstens wrote:
> On Fri, Apr 26, 2019 at 08:20:52PM -0400, Prarit Bhargava wrote:
>> Heiko and Jessica,
>>
>> The issue doesn't appear to be with my patch AFAICT.  The s390_trng fails to
>> load and then the kernel occasionally hangs (as Heiko mentioned) calling
>> synchronize_rcu().
>>
>> The call sequence is
>>
>> module_load()
>> 	do_init_module()
>> 		do_one_initcall(mod->init)
>>
>> which fails.
>>
>> The failure path in do_one_initcall() is entered and we start executing code at
>> kernel/module.c:3541
>>
>> fail_free_freeinit:
>>         kfree(freeinit);
>> fail:
>>         /* Try to protect us from buggy refcounters. */
>>         mod->state = MODULE_STATE_GOING;
>>         synchronize_rcu();
>>
>> ^^^ the kernel hangs here.  Sometimes it's very short and other times it seems
>> to hang.   I've left systems that appear to be hung and come back after 10
>> minutes to find that they've somehow made it through this call.
>>
>> Is there a known issue with RCU on s390 that is making this occur?
> 
> No there is no known issue with RCU on s390. The reason that
> synchronize_rcu() doesn't finish is because a different cpu is within
> an endless loop in add_unformed_module() just like Jessica suspected.
> 
> Note: the kernel is compiled with CONFIG_PREEMPT off - there is no
> kernel preemption that will make the looping cpu ever go over schedule
> and subsequently let synchronize_rcu() finish.
> 
> To confirm Jessicas theory - looking into the dump we have:
> 
> crash> bt 742
> PID: 742    TASK: 1efa6c000         CPU: 7   COMMAND: "systemd-udevd"
>  #0 [3e0043aba30] __schedule at abb25e
>  #1 [3e0043abaa0] schedule at abb6a2
>  #2 [3e0043abac8] schedule_timeout at abf49a
>  #3 [3e0043abb60] wait_for_common at abc396
>  #4 [3e0043abbf0] __wait_rcu_gp at 1c0136
>  #5 [3e0043abc48] synchronize_rcu at 1c72ea
>  #6 [3e0043abc98] do_init_module at 1f10be
>  #7 [3e0043abcf0] load_module at 1f3594
>  #8 [3e0043abdd0] __se_sys_init_module at 1f3af0
>  #9 [3e0043abea8] system_call at ac0766
> 
> Which is the process waiting for synchronize_rcu to finish. Wading
> through the stack frames gives me this struct module:
> 
> struct module {
>   state = MODULE_STATE_GOING,
>   list = {
>     next = 0x3ff80394508,
>     prev = 0xe25090 <modules>
>   },
>   name = "s390_trng\000...
> ...
> 
> Then we have the looping task/cpu:
> 
> PID: 731    TASK: 1e79ba000         CPU: 7   COMMAND: "systemd-udevd"
>  LOWCORE INFO:
>   -psw      : 0x0704c00180000000 0x0000000000ab666a
>   -function : memcmp at ab666a
> ...
>   -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
> 
> which is find_module_all() calling memcmp with this string:
> 
> 3ff80347318:  733339305f74726e 6700000000000000   s390_trng.......
> 
> So it all seems to fit. A simple cond_resched() call, which enforces
> an RCU quiescent state for the calliung cpu, fixes the problem for me
> (patch on top of linux-next 20190424 -- c392798a85ab):
> 
> diff --git a/kernel/module.c b/kernel/module.c
> index 410eeb7e4f1d..48748cfec991 100644
> --- a/kernel/module.c
> +++ b/kernel/module.c
> @@ -3585,6 +3585,7 @@ again:
>  					       finished_loading(mod->name));
>  			if (err)
>  				goto out_unlocked;
> +			cond_resched();

You just beat me to posting this :).  I am testing out this same patch in a loop
on a few systems here in the lab.  Will let you know results on Monday.

P.

>  			goto again;
>  		}
>  		err = -EEXIST;