[RFC,3/4] ntp: Use printk_deferred in leapsecond path

Message ID 1432931068-4980-4-git-send-email-john.stultz@linaro.org
State New
Headers show

Commit Message

John Stultz May 29, 2015, 8:24 p.m.
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.

Folks particularly worried about leapsecond crashes should
probably pay attention to this patch. Pending review, Its likely
a -stable candidate.

Cc: Prarit Bhargava <prarit@redhat.com>
Cc: Daniel Bristot de Oliveira <bristot@redhat.com>
Cc: Richard Cochran <richardcochran@gmail.com>
Cc: Jan Kara <jack@suse.cz>
Cc: Jiri Bohac <jbohac@suse.cz>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Shuah Khan <shuahkh@osg.samsung.com>
Signed-off-by: John Stultz <john.stultz@linaro.org>
---
 kernel/time/ntp.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

Comments

John Stultz June 2, 2015, 4:04 p.m. | #1
On Tue, Jun 2, 2015 at 3:31 AM, Jiri Bohac <jbohac@suse.cz> wrote:
> Hi,
>
> On Fri, May 29, 2015 at 01:24:27PM -0700, John Stultz 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.

Good point. The update is being done to the shadow-timekeeper, so we
won't block readers.  This can probably be dropped then. Although I'm
almost consider keeping it for consistency so I don't forget this
again and worry about it in the future.

thanks
-john
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/
John Stultz June 2, 2015, 4:14 p.m. | #2
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
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/

Patch hide | download patch | download mbox

diff --git a/kernel/time/ntp.c b/kernel/time/ntp.c
index 7a68100..472591e 100644
--- a/kernel/time/ntp.c
+++ b/kernel/time/ntp.c
@@ -393,7 +393,7 @@  int second_overflow(unsigned long secs)
 		else if (secs % 86400 == 0) {
 			leap = -1;
 			time_state = TIME_OOP;
-			printk(KERN_NOTICE
+			printk_deferred(KERN_NOTICE
 				"Clock: inserting leap second 23:59:60 UTC\n");
 		}
 		break;
@@ -403,7 +403,7 @@  int second_overflow(unsigned long secs)
 		else if ((secs + 1) % 86400 == 0) {
 			leap = 1;
 			time_state = TIME_WAIT;
-			printk(KERN_NOTICE
+			printk_deferred(KERN_NOTICE
 				"Clock: deleting leap second 23:59:59 UTC\n");
 		}
 		break;