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();
goto again;
}
err = -EEXIST;
