On Fri, May 30, 2014 at 10:13:28AM +0200, Jan Kara wrote: > On Thu 29-05-14 02:09:11, Frederic Weisbecker wrote: > > On Thu, May 29, 2014 at 12:02:30AM +0200, Jiri Kosina wrote: > > > On Fri, 9 May 2014, Petr Mladek wrote: > > > > > > > printk() cannot be used safely in NMI context because it uses internal > > > > locks > > > > and thus could cause a deadlock. Unfortunately there are circumstances > > > > when > > > > calling printk from NMI is very useful. For example, all > > > > WARN.*(in_nmi()) > > > > would be much more helpful if they didn't lockup the machine. > > > > > > > > Another example would be arch_trigger_all_cpu_backtrace for x86 which > > > > uses NMI > > > > to dump traces on all CPU (either triggered by sysrq+l or from RCU stall > > > > detector). > > > > > > I am rather surprised that this patchset hasn't received a single review > > > comment for 3 weeks. > > > > > > Let me point out that the issues Petr is talking about in the cover > > > letter > > > are real -- we've actually seen the lockups triggered by RCU stall > > > detector trying to dump stacks on all CPUs, and hard-locking machine up > > > while doing so. > > > > > > So this really needs to be solved. > > > > The lack of review may be partly due to a not very appealing changestat on > > an > > old codebase that is already unpopular: > > > > Documentation/kernel-parameters.txt | 19 +- > > kernel/printk/printk.c | 1218 > > +++++++++++++++++++++++++---------- > > 2 files changed, 878 insertions(+), 359 deletions(-) > > > > > > Your patches look clean and pretty nice actually. They must be seriously > > considered if we want to keep the current locked ring buffer design and > > extend it to multiple per context buffers. But I wonder if it's worth to > > continue that way with the printk ancient design. > > > > If it takes more than 1000 line changes (including 500 added) to make it > > finally work correctly with NMIs by working around its fundamental flaws, > > shouldn't we rather redesign it to use a lockless ring buffer like ftrace > > or perf ones? > I agree that lockless ringbuffer would be a more elegant solution but a > much more intrusive one and complex as well. Petr's patch set basically > leaves ordinary printk path intact to avoid concerns about regressions > there. > > Given how difficult / time consuming is it to push any complex changes to > printk I'd push for fixing printk from NMI in this inelegant but relatively > non-contentious way and work on converting printk to lockless > implementation long term. But before spending huge amount of time on that > I'd like to get some wider concensus that this is really the way we want to > go - at least AKPM and Steven - something for discussion in the KS topic I'd > proposed I think [1].
Agreed, lets wait for others opinion. Andrew, Steve? -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
