On Tue, 15 Dec 2015 15:26:21 +0100 Petr Mladek <pmladek@xxxxxxxx> 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.
