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    SubjectRe: [RFC PATCH 00/11] printk: safe printing in NMI context
    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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