linux-kernel - Re: [PATCH v11 3/3] printk: make printk.synchronous param rw

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Date:	Fri, 08 Apr 2016 13:44:50 +0800
From:	Pan Xinhui <xinhui@...ux.vnet.ibm.com>
To:	Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
CC:	Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Jan Kara <jack@...e.com>, Petr Mladek <pmladek@...e.com>,
	Tejun Heo <tj@...nel.org>,
	Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
	linux-kernel@...r.kernel.org,
	Byungchul Park <byungchul.park@....com>
Subject: Re: [PATCH v11 3/3] printk: make printk.synchronous param rw



On 2016年04月08日 13:29, Sergey Senozhatsky wrote:
> On (04/08/16 12:04), Pan Xinhui wrote:
> [..]
>>> +/*
>>> + * Init async printk via late_initcall, after core/arch/device/etc.
>>> + * initialization.
>>> + */
>>> +static __init int init_printk_kthread(void)
>>> +{
>>> +	printk_initcall_done = true;
>>> +	return __init_printk_kthread();
>> hello, 
>>
>> One confusion, Why not use a lock to protect __init_printk_kthread from parallel call? Otherwise I think there is a race.
>> But for simplicity, maybe you could write codes as below.
>>
>> +	int ret  = __init_printk_kthread();
>> +	printk_initcall_done = true;
>> +	return ret;
>>
>> In my opinion, using a lock is better.
> 
> Hello,
> 
> I though about this, but isn't late_initcall() happening before kernel
> starts /sbin/init? who can race with
> 
> 	late_initcall() -> init_printk_kthread() -> __init_printk_kthread()?
> 
yep, you are right. I made a mistake.

> looking at
> 
> static int __ref kernel_init(void *unused)
> {
>         int ret;
> 
>         kernel_init_freeable();
>         /* need to finish all async __init code before freeing the memory */
>         async_synchronize_full();
>         free_initmem();
> ..
> 
>         if (!try_to_run_init_process("/sbin/init") ||
>             !try_to_run_init_process("/etc/init") ||
>             !try_to_run_init_process("/bin/init") ||
>             !try_to_run_init_process("/bin/sh"))
>                 return 0;
> 
> __init (and init_printk_kthread is __init) is finished and freed by the
> time kernel try_to_run_init_process. isn't it?
> 
> 
good explanation. thanks

> sysfs knob -> __init_printk_kthread() is protected by printk_sync_lock
> mutex, obviously there can be parallel calls from user space.
> 
> 	-ss
>