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    SubjectRe: [RFC][PATCH 3/4] ntp: Use printk_deferred in leapsecond path
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    On Tue, Jun 2, 2015 at 3:43 AM, Jiri Kosina <jkosina@suse.cz> wrote:
    > On Tue, 2 Jun 2015, Jiri Bohac wrote:
    >
    >> > Looking over the leapsecond code, I noticed the printk messages
    >> > reporting the leapsecond insertion in the second_overflow path
    >> > were not using the printk_deferred method. This was surprising
    >> > since the printk_deferred method was added in part to avoid
    >> > printk-ing while holding the timekeeping locks.
    >> >
    >> > See 6d9bcb621b0b (timekeeping: use printk_deferred when holding
    >> > timekeeping seqlock) for further rational.
    >> >
    >> > I can only guess that this omission was a git add -p oversight.
    >>
    >> second_overflow() is called from accumulate_nsecs_to_secs().
    >>
    >> accumulate_nsecs_to_secs() is called from update_wall_time()
    >> - once directly
    >> - once via logarithmic_accumulation()
    >> Both calls are before write_seqcount_begin(&tk_core.seq).
    >>
    >> So it looks safe to use printk there.
    >
    > Couldn't we stuff a couple of
    >
    > !lockdep_is_held()
    >
    > assertions into printk() so that we don't have to keep rediscovering this
    > sort of problems over and over again?

    Yea. I was thinking if we could add something very early in printk
    before we disable lockdep where we lockdep_aquire/release a few of the
    locks we know printk might take, it would help close the gap on these
    sorts of call paths that surprise us.

    Lockdep is *such* a great tool, because it provides some confidence
    that changes don't cause locking regressions, so to have printk poke a
    hole in that confidence is frustrating.

    thanks
    -john


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