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[Dan Streetman <dan.streetman@xxxxxxxxxxxxx>]

this is (mostly) fixed in 2 commits, from the original bug (bug 1505564)
kernel level commit 5a3a466ae3c86ad6b8fc36c2efc5f541ef0ed89e (from
git://kernel.ubuntu.com/ubuntu/ubuntu-trusty.git lts-backport-vivid)
partially fixes this problem, by using the console_may_schedule var
correctly.  The recent upstream commit
6b97a20d3a7909daa06625d4440c2c52d7bf08d7 (from upstream) completes
fixing the problem, by setting console_may_schedule correctly.

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https://bugs.launchpad.net/bugs/1534216

Title:
  lots of printk to serial console can hang system for long time

Status in linux package in Ubuntu:
  In Progress

Bug description:
  This is a clone from bug 1505564, to track the separate issue of the
  serial port driver failing to schedule itself off its cpu.

  The original bug's problem was caused by the kernel spamming a huge
  number of error messages in a certain situation.  Normally, that would
  not be a problem, but in this case the system is virtualized, and logs
  over its serial port.  When the massive number of kernel messages are
  sent to the serial port driver, it can't keep up, so sending all the
  log messages can take a very long time - minutes or longer - and the
  serial port driver fails to schedule itself off the cpu it's using
  during that time.  That results in other cpus hanging, waiting for the
  serial port driver's cpu to become avaiable.

  I'll  update the bug with more details as I debug.

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