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
