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?
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()
...
-ss
