[lttng-dev] [tip:timers/urgent] timekeeping: Fix HRTICK related deadlock from ntp lock changes

[Mathieu Desnoyers]

Hi Ingo,

Do you have an estimate of the time it will take for this fix to hit
mainline, stable-3.10 and stable-3.11 ? Meanwhile, I'm marking 3.10 and
3.11 as broken for LTTng with a kernel version at compile-time, since
this kernel regression currently triggers hard system lockup when people
use LTTng on those kernels, and this is certainly something nobody
wants.

Thanks,

Mathieu

* tip-bot for John Stultz (tipbot at zytor.com) wrote:
> Commit-ID:  7bd36014460f793c19e7d6c94dab67b0afcfcb7f
> Gitweb:     http://git.kernel.org/tip/7bd36014460f793c19e7d6c94dab67b0afcfcb7f
> Author:     John Stultz <john.stultz at linaro.org>
> AuthorDate: Wed, 11 Sep 2013 16:50:56 -0700
> Committer:  Ingo Molnar <mingo at kernel.org>
> CommitDate: Thu, 12 Sep 2013 07:49:51 +0200
> 
> timekeeping: Fix HRTICK related deadlock from ntp lock changes
> 
> Gerlando Falauto reported that when HRTICK is enabled, it is
> possible to trigger system deadlocks. These were hard to
> reproduce, as HRTICK has been broken in the past, but seemed
> to be connected to the timekeeping_seq lock.
> 
> Since seqlock/seqcount's aren't supported w/ lockdep, I added
> some extra spinlock based locking and triggered the following
> lockdep output:
> 
> [   15.849182] ntpd/4062 is trying to acquire lock:
> [   15.849765]  (&(&pool->lock)->rlock){..-...}, at: [<ffffffff810aa9b5>] __queue_work+0x145/0x480
> [   15.850051]
> [   15.850051] but task is already holding lock:
> [   15.850051]  (timekeeper_lock){-.-.-.}, at: [<ffffffff810df6df>] do_adjtimex+0x7f/0x100
> 
> <snip>
> 
> [   15.850051] Chain exists of: &(&pool->lock)->rlock --> &p->pi_lock --> timekeeper_lock
> [   15.850051]  Possible unsafe locking scenario:
> [   15.850051]
> [   15.850051]        CPU0                    CPU1
> [   15.850051]        ----                    ----
> [   15.850051]   lock(timekeeper_lock);
> [   15.850051]                                lock(&p->pi_lock);
> [   15.850051] lock(timekeeper_lock);
> [   15.850051] lock(&(&pool->lock)->rlock);
> [   15.850051]
> [   15.850051]  *** DEADLOCK ***
> 
> The deadlock was introduced by 06c017fdd4dc48451a ("timekeeping:
> Hold timekeepering locks in do_adjtimex and hardpps") in 3.10
> 
> This patch avoids this deadlock, by moving the call to
> schedule_delayed_work() outside of the timekeeper lock
> critical section.
> 
> Reported-by: Gerlando Falauto <gerlando.falauto at keymile.com>
> Tested-by: Lin Ming <minggr at gmail.com>
> Signed-off-by: John Stultz <john.stultz at linaro.org>
> Cc: Mathieu Desnoyers <mathieu.desnoyers at efficios.com>
> Cc: stable <stable at vger.kernel.org> #3.11, 3.10
> Link: http://lkml.kernel.org/r/1378943457-27314-1-git-send-email-john.stultz@linaro.org
> Signed-off-by: Ingo Molnar <mingo at kernel.org>
> ---
>  include/linux/timex.h     | 1 +
>  kernel/time/ntp.c         | 6 ++----
>  kernel/time/timekeeping.c | 2 ++
>  3 files changed, 5 insertions(+), 4 deletions(-)
> 
> diff --git a/include/linux/timex.h b/include/linux/timex.h
> index b3726e6..dd3edd7 100644
> --- a/include/linux/timex.h
> +++ b/include/linux/timex.h
> @@ -141,6 +141,7 @@ extern int do_adjtimex(struct timex *);
>  extern void hardpps(const struct timespec *, const struct timespec *);
>  
>  int read_current_timer(unsigned long *timer_val);
> +void ntp_notify_cmos_timer(void);
>  
>  /* The clock frequency of the i8253/i8254 PIT */
>  #define PIT_TICK_RATE 1193182ul
> diff --git a/kernel/time/ntp.c b/kernel/time/ntp.c
> index 8f5b3b9..bb22151 100644
> --- a/kernel/time/ntp.c
> +++ b/kernel/time/ntp.c
> @@ -516,13 +516,13 @@ static void sync_cmos_clock(struct work_struct *work)
>  	schedule_delayed_work(&sync_cmos_work, timespec_to_jiffies(&next));
>  }
>  
> -static void notify_cmos_timer(void)
> +void ntp_notify_cmos_timer(void)
>  {
>  	schedule_delayed_work(&sync_cmos_work, 0);
>  }
>  
>  #else
> -static inline void notify_cmos_timer(void) { }
> +void ntp_notify_cmos_timer(void) { }
>  #endif
>  
>  
> @@ -687,8 +687,6 @@ int __do_adjtimex(struct timex *txc, struct timespec *ts, s32 *time_tai)
>  	if (!(time_status & STA_NANO))
>  		txc->time.tv_usec /= NSEC_PER_USEC;
>  
> -	notify_cmos_timer();
> -
>  	return result;
>  }
>  
> diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
> index 48b9fff..947ba25 100644
> --- a/kernel/time/timekeeping.c
> +++ b/kernel/time/timekeeping.c
> @@ -1703,6 +1703,8 @@ int do_adjtimex(struct timex *txc)
>  	write_seqcount_end(&timekeeper_seq);
>  	raw_spin_unlock_irqrestore(&timekeeper_lock, flags);
>  
> +	ntp_notify_cmos_timer();
> +
>  	return ret;
>  }
>  

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com