Re: [PATCH] printk/nmi: avoid direct printk()-s from __printk_nmi_flush()

[Petr Mladek <pmladek@suse.com>]

On Tue 2016-08-30 18:39:18, Sergey Senozhatsky wrote:
> On (08/30/16 11:04), Petr Mladek wrote:
> > On Tue 2016-08-30 16:58:34, Sergey Senozhatsky wrote:
> > > Petr,
> > > one more question. Not related to the patch, but still related to NMI.
> > > 
> > > can NMI nest?
> > 
> > AFAIK, they cannot. NMIs should be disabled until iret is called.
> > Therefore we should be on the safe side if iret is not called
> > inside the NMI handler. But this should not happen because
> > it would cause other problems, like using wrong return address.
> > 
> > Well, x86 nmi code has some hacks to handle exceptions inside
> > NMI handlers that use iret. But printk_nmi_enter()/printk_nmi_exit()
> > are never nested there. It is prevented by the nmi_state per-CPU
> > variable. See do_nmi() in arch/x86/kernel/nmi.c.
> 
> yes, x86 has a per-cpu nmi_state to handle the case when NMI is
> loosing its NMI context. But other arch-s, as far as I can see,
> don't do that. Does it mean that we are safe only on x86?

My understanding is that the kernel would crash on the other
architectures if a double iret was called. By other words,
they would have bigger problems than the nmi_enter()/nmi_exit()
calls. So, we should be on the safe side.

> this printk_func_saved thing is still will be needed, I think,
> for alt_printk.
> 
> Example:
> 
> process abc
> 	printk()
> 		alt_printk_enter()
> 			this_cpu_write(printk_func, vprintk_alt);
> ->	NMI
> 	:	printk_nmi_enter()
> 	:		this_cpu_write(printk_func, vprintk_nmi);
> 	:	printk_nmi_exit()
> 	:		this_cpu_write(printk_func, vprintk_default);
> 	return NMI
> 
> 		printk()  <<<<  nested printk -> vprintk_default(), set by nmi_exit()
> 		alt_printk_exit()
> 	...

I see. But then we will need to be more careful because printk_func
and printk_func_saved will be manipulated in different contexts:
normal, irq, nmi. A solution might be using an atomic counter
and selecting the right vprintk_func according to the value.

Well, I am still afraid that yet another alt_printk is not
the way to go.

Best Regards,
Petr