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    SubjectRe: [PATCH] v2 Re: Bug: Status/Summary of slashdot leap-second crash on new years 2008-2009
    Once upon a time, Duane Griffin <duaneg@dghda.com> said:
    > How about instead of a switch statement, assigning the message to a
    > variable and printing that. I.e. something like:

    Good point. Here's an updated version that also adds a comment to the
    xtime_lock definition about not using printk.
    --
    Chris Adams <cmadams@hiwaay.net>
    Systems and Network Administrator - HiWAAY Internet Services
    I don't speak for anybody but myself - that's enough trouble.


    From: Chris Adams <cmadams@hiwaay.net>

    The code to handle leap seconds printks an information message when the
    second is inserted or deleted. It does this while holding xtime_lock.
    However, printk wakes up klogd, and in some cases, the scheduler tries
    to get the current kernel time, trying to get xtime_lock (which results
    in a deadlock). This moved the printks outside of the lock. It also
    adds a comment to not use printk while holding xtime_lock.

    Signed-off-by: Chris Adams <cmadams@hiwaay.net>
    ---
    diff -urpN linux-2.6.28-git5-vanilla/include/linux/time.h linux-2.6.28-git5/include/linux/time.h
    --- linux-2.6.28-git5-vanilla/include/linux/time.h 2009-01-02 22:09:10.000000000 -0600
    +++ linux-2.6.28-git5/include/linux/time.h 2009-01-03 11:57:27.000000000 -0600
    @@ -99,6 +99,12 @@ static inline struct timespec timespec_s

    extern struct timespec xtime;
    extern struct timespec wall_to_monotonic;
    +
    +/*
    + * Do not call printk while holding this lock; it wakes klogd and the
    + * scheduler may try to get the current kernel time, which will try to get
    + * this lock.
    + */
    extern seqlock_t xtime_lock;

    extern unsigned long read_persistent_clock(void);
    diff -urpN linux-2.6.28-git5-vanilla/kernel/time/ntp.c linux-2.6.28-git5/kernel/time/ntp.c
    --- linux-2.6.28-git5-vanilla/kernel/time/ntp.c 2009-01-02 22:09:34.000000000 -0600
    +++ linux-2.6.28-git5/kernel/time/ntp.c 2009-01-03 11:57:46.000000000 -0600
    @@ -130,6 +130,7 @@ void ntp_clear(void)
    static enum hrtimer_restart ntp_leap_second(struct hrtimer *timer)
    {
    enum hrtimer_restart res = HRTIMER_NORESTART;
    + const char *msg = NULL;

    write_seqlock(&xtime_lock);

    @@ -140,8 +141,7 @@ static enum hrtimer_restart ntp_leap_sec
    xtime.tv_sec--;
    wall_to_monotonic.tv_sec++;
    time_state = TIME_OOP;
    - printk(KERN_NOTICE "Clock: "
    - "inserting leap second 23:59:60 UTC\n");
    + msg = "Clock: inserting leap second 23:59:60 UTC";
    hrtimer_add_expires_ns(&leap_timer, NSEC_PER_SEC);
    res = HRTIMER_RESTART;
    break;
    @@ -150,8 +150,7 @@ static enum hrtimer_restart ntp_leap_sec
    time_tai--;
    wall_to_monotonic.tv_sec--;
    time_state = TIME_WAIT;
    - printk(KERN_NOTICE "Clock: "
    - "deleting leap second 23:59:59 UTC\n");
    + msg = "Clock: deleting leap second 23:59:59 UTC";
    break;
    case TIME_OOP:
    time_tai++;
    @@ -166,6 +165,9 @@ static enum hrtimer_restart ntp_leap_sec

    write_sequnlock(&xtime_lock);

    + if (msg)
    + printk(KERN_NOTICE "%s\n", msg);
    +
    return res;
    }


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