linux-kernel - is printk() safe within a timekeeper

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Date:	Thu, 6 Mar 2014 18:45:31 +0100
From:	Jiri Bohac <jbohac@...e.cz>
To:	John Stultz <john.stultz@...aro.org>, linux-kernel@...r.kernel.org
Cc:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: is printk() safe within a timekeeper_seq write section?

Hi,

I'm looking at the printk call in
__timekeeping_inject_sleeptime(), introduced in cb5de2f8
(time: Catch invalid timespec sleep values in __timekeeping_inject_sleeptime)

Is it safe to call printk() while timekeeper_seq is held for
writing?

What about this call chain?
  printk
    vprintk_emit
      console_unlock
        up(&console_sem)
          __up
	    wake_up_process
	      try_to_wake_up
	        ttwu_do_activate
		  ttwu_activate
		    activate_task
		      enqueue_task
		        enqueue_task_fair
			  hrtick_update
			    hrtick_start_fair
			      hrtick_start_fair
			        get_time
				  ktime_get
				    --> endless loop on
				    read_seqcount_retry(&timekeeper_seq, ...)

It looks like an unlikely but possible deadlock. 
Or did I overlook something?

Thanks!

-- 
Jiri Bohac <jbohac@...e.cz>
SUSE Labs, SUSE CZ

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