Messages in this thread | | | Date | Thu, 29 May 2014 10:09:48 +0200 (CEST) | From | Jiri Kosina <> | Subject | Re: [RFC PATCH 00/11] printk: safe printing in NMI context |
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On Thu, 29 May 2014, Frederic Weisbecker wrote:
> > 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?
Yeah, printk() has grown over years to a stinking pile of you-know-what, no argument to that.
I also agree that performing a massive rewrite, which will make it use a lockless buffer, and therefore ultimately solve all its problems (scheduler deadlocks, NMI deadlocks, xtime_lock deadlocks) at once, is necessary in the long run.
On the other hand, I am completely sure that the diffstat for such rewrite is going to be much more scary :)
This is not adding fancy features to printk(), where we really should be saying no; horrible commits like 7ff9554bb5 is exactly something that should be pushed against *heavily*. But bugfixes for hard machine lockups are a completely different story to me (until we have a whole new printk() buffer handling implementation).
-- Jiri Kosina SUSE Labs
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