Re: [PATCH] v2 Re: Bug: Status/Summary of slashdot leap-second crash on new years 2008-2009

Chris Adams
2009-01-03T18:02:10Z

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;
}