linux-kernel - Re: [PATCH printk v5 1/1] printk: extend console

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Message-ID: <601d6979-7462-6f20-0d60-2fcfcf1e405b@samsung.com>
Date:   Tue, 3 May 2022 00:29:00 +0200
From:   Marek Szyprowski <m.szyprowski@...sung.com>
To:     John Ogness <john.ogness@...utronix.de>,
        Petr Mladek <pmladek@...e.com>
Cc:     Sergey Senozhatsky <senozhatsky@...omium.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel@...r.kernel.org,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-amlogic@...ts.infradead.org
Subject: Re: [PATCH printk v5 1/1] printk: extend console_lock for
 per-console locking

Hi John,

On 02.05.2022 15:11, John Ogness wrote:
> On 2022-05-02, Marek Szyprowski <m.szyprowski@...sung.com> wrote:
>> Data written to /dev/kmsg and all kernel logs were always displayed
>> correctly. Also data written directly to /dev/ttyAML0 is displayed
>> properly on the console. The latter doesn't however trigger the input
>> related activity.
>>
>> It looks that the data read from the uart is delivered only if other
>> activity happens on the kernel console. If I type 'reboot' and press
>> enter, nothing happens immediately. If I type 'date >/dev/ttyAML0' via
>> ssh then, I only see the date printed on the console. However if I
>> type 'date >/dev/kmsg', the the date is printed and reboot happens.
> I suppose if you login via ssh and check /proc/interrupts, then type
> some things over serial, then check /proc/interrupts again, you will see
> there have been no interrupts for the uart. But interrupts for other
> devices are happening. Is this correct?

Right. The counter for ttyAML0 is not increased when lockup happens and 
I type something to the uart console.

>> For comparison, here is a 't' sysrq result from the 'working' serial
>> console (next-20220429), which happens usually 1 of 4 boots:
>>
>> https://protect2.fireeye.com/v1/url?k=3ef0fd63-5f7be855-3ef1762c-000babff9b5d-2e40dc5adc30a14c&q=1&e=1469838f-8586-403e-bd4d-922675d8b658&u=https%3A%2F%2Fpastebin.com%2Fmp8zGFbW
> This still looks odd to me. We should be seeing a trace originating from
> ret_from_fork+0x10/0x20 and kthread+0x118/0x11c.
>
> I wonder if the early creation of the thread is somehow causing
> problems. Could you try the following patch to see if it makes a
> difference? I would also like to see the sysrq-t output with this patch
> applied:
>
> ---------------- BEGIN PATCH ---------------
> diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
> index 2311a0ad584a..c4362d25de22 100644
> --- a/kernel/printk/printk.c
> +++ b/kernel/printk/printk.c
> @@ -3837,7 +3837,7 @@ static int __init printk_activate_kthreads(void)
>   
>   	return 0;
>   }
> -early_initcall(printk_activate_kthreads);
> +late_initcall(printk_activate_kthreads);
>   
>   #if defined CONFIG_PRINTK
>   /* If @con is specified, only wait for that console. Otherwise wait for all. */
> ---------------- END PATCH ---------------
>
> Thanks for your help with this!

Well, nothing has changed. The lockup still happens.

Best regards
-- 
Marek Szyprowski, PhD
Samsung R&D Institute Poland