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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [thread-next>] [day] [month] [year] [list]

Message-ID: <20140306174531.GA30634@midget.suse.cz>
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

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/