Re: [linux-next:master] [serial] b63e6f60ea: BUG:soft_lockup-CPU##stuck_for#s![modprobe:#]

[John Ogness <john.ogness@linutronix.de>]

On 2025-03-15, Ryo Takakura <ryotkkr98@gmail.com> wrote:
> Is the thread still active?

Yes. It is one of the open issues that needs to be resolved before I
repost the 8250 atomic console series.

> I got the same softlockup during the test regardless of the presence
> of the commits.
>
> [   60.222013] watchdog: BUG: soft lockup - CPU#2 stuck for 22s! [swapper/0:1]
> [   60.222023] Modules linked in:
> [   60.222032] CPU: 2 UID: 0 PID: 1 Comm: swapper/0 Tainted: G             L     6.14.0-rc6-v14-rc6-voluntary+ #4
> [   60.222047] Tainted: [L]=SOFTLOCKUP
> [   60.222051] Hardware name: Raspberry Pi 4 Model B Rev 1.5 (DT)
> [   60.222055] pstate: 60000005 (nZCv daif -PAN -UAO -TCO -DIT -SSBS BTYPE=--)
> [   60.222066] pc : get_random_u32+0xac/0x118
> [   60.222081] lr : __get_random_u32_below+0x20/0x78
> [   60.222094] sp : ffffffc08002bb80
> [   60.222098] x29: ffffffc08002bb80 x28: 0000000000000003 x27: 0000000000000001
> [   60.222114] x26: ffffff804112e6a4 x25: ffffffd33ed21820 x24: ffffff804112e69c
> [   60.222128] x23: 0000000000000000 x22: ffffff804112e64e x21: 0000000000000000
> [   60.222142] x20: 000000000000000d x19: ffffff80fb7aebb8 x18: 0000000000000002
> [   60.222156] x17: 0000000000000004 x16: ffffff804112e584 x15: ffffff8041126796
> [   60.222169] x14: ffffff80411267c0 x13: 0000000000000006 x12: ffffff804112e5c0
> [   60.222183] x11: ffffff804112e64c x10: 0000000000000007 x9 : ffffffd33dccdd10
> [   60.222196] x8 : ffffff804112e6a8 x7 : 0000000000000000 x6 : 0005000400060005
> [   60.222210] x5 : ffffff804112e65a x4 : 0000000000000000 x3 : 0000000000000010
> [   60.222223] x2 : 0000000000000014 x1 : 000000002c7d0b7a x0 : 0000000000000013
> [   60.222237] Call trace:
> [   60.222241]  get_random_u32+0xac/0x118 (P)
> [   60.222256]  __get_random_u32_below+0x20/0x78
> [   60.222268]  get_rcw_we+0x180/0x208
> [   60.222278]  test_rslib_init+0x2c8/0xba0
> [   60.222292]  do_one_initcall+0x4c/0x210
> [   60.222303]  kernel_init_freeable+0x1fc/0x3a0
> [   60.222317]  kernel_init+0x28/0x1f8
> [   60.222327]  ret_from_fork+0x10/0x20
>
>> I wonder if a cond_resched() in some loop would help. Or using a
>
> I wasn't sure which loop would be the appropriate one but adding
> cond_resched() as below worked as suggested.
>
> ----- BEGIN -----
> diff --git a/lib/reed_solomon/test_rslib.c b/lib/reed_solomon/test_rslib.c
> index 75cb1adac..322d7b0a8 100644
> --- a/lib/reed_solomon/test_rslib.c
> +++ b/lib/reed_solomon/test_rslib.c
> @@ -306,6 +306,8 @@ static void test_uc(struct rs_control *rs, int len, int errs,
>
>                 if (memcmp(r, c, len * sizeof(*r)))
>                         stat->dwrong++;
> +
> +               cond_resched();
>         }
>         stat->nwords += trials;
>  }
> @@ -400,6 +402,8 @@ static void test_bc(struct rs_control *rs, int len, int errs,
>                 } else {
>                         stat->rfail++;
>                 }
> +
> +               cond_resched();
>         }
>         stat->nwords += trials;
>  }
> ----- END -----
>
>> pseudorandom generator. I remember the problems related to much
>> slower random generator, for example, see the commit
>> f900fde28883602b6 ("crypto: testmgr - fix RNG performance in fuzz
>> tests").
>
> I haven't tested this but I'll look into it!
>
>> That said, I did not dig deep into the code. And I did not try to
>> reproduce the softlockup. I am pretty busy at the moment with some
>> other stuff. I just wanted to give it a look and share my opinion.
>
> I think the softlockup is rather a problem of test itself, 
> not the two commits.

Thanks Ryo for looking into this! I think we need to have a technical
explanation/understanding of the problem so that it is clear how my
series triggers or exaggerates the issue.

John