[PATCH v3 4/4] printk/nmi: Increase the size of NMI buffer and make it configurable

[Andrew Morton]

On Tue, 15 Dec 2015 15:26:21 +0100 Petr Mladek <pmladek at suse.com> wrote:

> > OK, thanks.  So "not needed at present, might be needed in the future,
> > useful for out-of-tree debug code"?
> 
> It is possible that I got it a wrong way on arm. The NMI buffer is
> usable there on two locations.
> 
> First, the temporary is currently used to handle IPI_CPU_BACKTRACE.
> It seems that it is not a real NMI. But it seems to be available
> (compiled) on all arm system. This is why I introduced NEED_PRINTK_NMI
> Kconfig flag to avoid confusion with a real NMI.
> 
> Second, there is the FIQ "NMI" handler that is called from
> /arch/arm/kernel/entry-armv.S. It is compiled only if _not_
> defined $(CONFIG_CPU_V7M). It calls nmi_enter() and nmi_stop().
> It looks like a real NMI handler. This is why I defined HAVE_NMI
> if (!CPU_V7M).
> 
> A solution would be to define HAVE_NMI on all Arm systems and get rid
> of NEED_PRINTK_NMI. If you think that it would cause less confusion...

So does this mean that the patch will be updated?

> 
> > > there's this effort to apply further cleanups - to me, the changelogs
> > > don't seem to make that much sense, unless we want to start using
> > > printk() extensively in NMI functions - using the generic nmi backtrace
> > > code surely gets us something that works across all architectures...
> > 
> > Yes, I was scratching my head over that.  The patchset takes an nmi-safe
> > all-cpu-backtrace and generalises that into an nmi-safe printk.  That
> > *sounds* like a good thing to do but yes, some additional justification
> > would be helpful.  What real-world value does this patchset really
> > bring to real-world users?
> 
> The patchset brings two big advantages. First, it makes the NMI
> backtraces safe on all architectures for free. Second, it makes
> all NMI messages almost[*] safe on all architectures.
> 
> Note that there already are several messages printed in NMI context.
> See the mail from Jiri Kosina. They are not easy to avoid.
> 
> [*] The temporary buffer is limited. We still should keep
> the number of messages in NMI context at minimum.

This is important info - in fact a paragraph which starts with "The
patchset brings two big advantages" is *the most* important info.  I
added the below text to the [1/n] changelog:

: The patchset brings two big advantages.  First, it makes the NMI
: backtraces safe on all architectures for free.  Second, it makes all NMI
: messages almost safe on all architectures (the temporary buffer is
: limited.  We still should keep the number of messages in NMI context at
: minimum).
: 
: Note that there already are several messages printed in NMI context:
: WARN_ON(in_nmi()), BUG_ON(in_nmi()), anything being printed out from MCE
: handlers.  These are not easy to avoid.