[PATCH 0/4] Convert timekeeping core to use printk_deferred (v3)

[John Stultz]

Recently, Jiri pointed out a potential deadlock when calling printk
while holding the timekeeping seqlock.

Annoyingly, the seqlock lockdep enablement doesn't catch this, as
printk disables lockdep.

When looking for possible solutions, one idea was to use a local buffer
and defer the printk to later. Ends up there is already similar
functionality in printk_sched() to avoid similar style deadlocks w/
the scheduler.

Thus this patchset (based on next/akpm) renames printk_sched to
printk_deferred and then moves the affected timekeeping printks to make
use of it.

There were some points in the discussion between Jan and Peter that
made it seem that there may still be problems lurking in the console
layer, and I'm not sure I fully understand their point, so this solution
may be incomplete.

Additionally, the same issue likely affects any WARN_ONs as well, but
I wanted to get some thoughts on this approach before trying to remove
or convert affected WARN_ONS.

Your thoughts and feedback are greatly appreciated!

thanks
-john

Changes since v2:
* Renamed printk_once_deferred() to printk_deferred_once(), per
  Andrew's suggestion
* Changed adding irqsave/restore to preempt_disable/enable, per
  Jan's suggestion

Changes since v1:
* Rebased on next/akpm, since there are queued prink patches there
* Re-added irqsave/restore per irc discussion w/ PeterZ 


Cc: Jan Kara <jack@xxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Jiri Bohac <jbohac@xxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>


John Stultz (4):
  printk: Disable preemption for printk_sched
  printk: Rename printk_sched to printk_deferred
  printk: Add printk_deferred_once
  timekeeping: Use printk_deferred when holding timekeeping seqlock

 include/linux/printk.h    | 17 ++++++++++++++---
 kernel/printk/printk.c    |  4 +++-
 kernel/sched/core.c       |  2 +-
 kernel/sched/deadline.c   |  7 +------
 kernel/sched/rt.c         |  8 +-------
 kernel/time/ntp.c         | 15 +++++++++------
 kernel/time/timekeeping.c |  7 ++++---
 7 files changed, 33 insertions(+), 27 deletions(-)

-- 
1.9.1

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