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

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Date:	Fri, 22 Apr 2016 10:41:06 +0200
From:	Petr Mladek <pmladek@...e.com>
To:	Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Cc:	Pan Xinhui <xinhui@...ux.vnet.ibm.com>,
	Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Jan Kara <jack@...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 Fri 2016-04-22 10:28:26, Sergey Senozhatsky wrote:
> Hello,
> 
> On (04/21/16 13:07), Petr Mladek wrote:
> [..]
> > Please, what is the purpose of "printk_initcall_done" then? If I get
> > this correctly, it will always be true when printk_sync_set() is
> > called.
> 
> well, this is a bit ugly, yes. kernel_param_ops defines ->set callback
> as printk_sync_set(). and this ->set callback is getting called from 2
> different paths (but it's really about underlying __init_printk_kthread()).
> 
> __init_printk_kthread() can be executed from:

Ah, I see and feel shame. It is actually explained in the comment
above printk_initcall_done declaration. Well, the explanation confused
me a bit ;-) I suggest to change it sligtly:

/*
 * printk_sync_set() can be called from two places:
 *
 * - early from start_kernel()->parse_args().  But we can't kthread_run()
 *   at this stage, so we just set the param value.  The actual
 *   initalization happens later, from the late_initcall().
 *
 * - even later from user space via sysfs knob.  We can kthread_run()
 *   there if needed.
 */

Or we could write this even more explicitely:

/*
 * Prevent starting kthreads from start_kernel()->parse_args().  It is not
 * possible at this stage.  Instead do so via the inticall.
 */
static bool printk_initcall_done;

In each case, I would move the comment and the declaration right above the
printk_sync_set().

> alternatively, I had this idea to re-define ->set() callback in init_printk_kthread().
> 
> IOW, by default we use param_set_bool(), so parse_args() will not cause any
> problems:
> 
>  static /*** can't 'const' anymore ***/ struct kernel_param_ops param_ops_printk_sync = {
>         .set = param_set_bool,
>         .get = param_get_bool,
>  };
> 
> and change it to printk_sync_set() in:
> 
> static __init int init_printk_kthread(void)
> {
> 	param_ops_printk_sync.set = printk_sync_set;
> 	return __init_printk_kthread();
> }
> 
> but I think that this bool flag is simpler to understand after all.

In addition, there would be problems to handle a possible change via
the sysfs knob. The bool flag looks much better to me :-)

Thanks a lot for detailed explanation.

Best Regards,
Petr