On (08/30/16 13:19), Petr Mladek wrote:
[..]
> > 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.
yes, I thought about something like this.
... or we can use the one and only 'nmi_seq' buffer and share it between
NMI and alt_printk, adding a special prefix to every message
if (in_nmi())
sprintf("NMI:%s", message)
else
sprintf("%s", message)
so, yes, it can get hairy, but at least it will be grep-able, still
better than nothing.
> Well, I am still afraid that yet another alt_printk is not
> the way to go.
well, it might be and it might be not.
-ss
