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Date: Thu, 15 Mar 2012 15:58:20 +0100 (CET)
From: Thomas Gleixner <tglx@linutronix.de>
To: Sasha Levin <levinsasha928@gmail.com>
cc: John Stultz <john.stultz@linaro.org>, Dave Jones <davej@redhat.com>,
        "linux-kernel@vger.kernel.org List" <linux-kernel@vger.kernel.org>,
        Peter Zijlstra <peterz@infradead.org>
Subject: Re: ntp: BUG: spinlock lockup on CPU#1
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On Thu, 15 Mar 2012, Thomas Gleixner wrote:
> CPU 0                                                    CPU 1
> do_adjtimex()
>   spin_lock_irq(&ntp_lock);
>     process_adjtimex_modes();				 timer_interrupt()
>       process_adj_status();                                do_timer()
>         ntp_start_leap_timer();                             write_lock(&xtime_lock);
>           hrtimer_start();                                  update_wall_time();
>              hrtimer_reprogram();                            ntp_tick_length()
>                tick_program_event()                            spin_lock(&ntp_lock);
>                  clockevents_program_event()
> 		   ktime_get()
>                      seq = req_seqbegin(xtime_lock);
> 
> So CPU0 waits for xtime seqence to become valid and CPU1 cant make
> progress because it's stuck on ntp_lock.

The same problem exists with ntp_clear() being called under the
xtime/timekeeper lock write locked. Nasty ...