Re: [PATCH v10 1/2] printk: Make printk() completely async

[Petr Mladek <pmladek@suse.com>]

On Mon 2016-08-22 13:15:20, Sergey Senozhatsky wrote:
> Hello,
> 
> On (08/20/16 14:24), Sergey Senozhatsky wrote:
> > On (08/19/16 21:00), Jan Kara wrote:
> > > > > depending on .config BUG() may never return back -- passing control
> > > > > to do_exit(), so printk_deferred_exit() won't be executed. thus we
> > > > > probably need to have a per-cpu variable that would indicate that
> > > > > we are in deferred_bug. hm... but do we really need deferred BUG()
> > > > > in the first place?
> > > > 
> > > > Good question. I am not aware of any BUG_ON() that would be called from
> > > > wake_up_process() but it is hard to check everything.
> > > > 
> > > > A conservative approach would be to force synchronous printk from
> > > > BUG_ON().
> > > 
> > > Just a quick thought: Cannot we just do printk_deferred_enter() when we are
> > > about to call into the scheduler from printk code and printk_deferred_exit()
> > > when leaving it? That would look like the least error-prone way how
> > > handling this kind of recursion...
> > 
> > interesting idea.
> > printk_deferred_enter() increments preempt count, so there may be additional
> > obstacles and, as a result, ad-hocs, that scheduler people will sincerely hate.
> > need to think more.
> 
> the other thing I just thought of is doing something as follows
> !!!not tested, will not compile, just an idea!!!
> 
> ---
> 
> diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
> index 6e260a0..bb8d719 100644
> --- a/kernel/printk/printk.c
> +++ b/kernel/printk/printk.c
> @@ -1789,6 +1789,7 @@ asmlinkage int vprintk_emit(int facility, int level,
>         printk_delay();
>  
>         local_irq_save(flags);
> +       printk_nmi_enter();
>         this_cpu = smp_processor_id();

Huh, this looks very interesting but I am afraid that it will not fly.
The problem is that vprintk_nmi() is safe only when it is used
exclusively in NMI.

The following could happen with your code:

/**** normar context ****/
vprintk_emit()
  printk_nmi_enter()
  ...
  wake_up_process()
    WARN()
      printk()
	vprintk_nmi()
	  vsnprintf(..., "0123456789")

	    /* real NMI comes after writing "01234" */
	    /**** NMI context ****/
	    vprintk_nmi();
	      vsnprintf(..., "abcdefghijklmno");

	    /**** normal context ****/
	    /* we finish writing "56789" into the buffer */

=> part of the message from NMI gets broken "abcde56789klmno".

The lockless handling of the NMI per-CPU buffer already is not
trivial. I would be afraid to add more hacks to make
it writable in all contexts.

I am sorry about the bad news. This was so promising on the first
look.

Best Regards,
Petr