Merge branch 'for-3.11-fixes' of git://git.kernel.org/pub/scm/linux/kernel/git/tj/wq

Pull workqueue fix from Tejun Heo:
 "This contains one fix which could lead to system-wide lockup on
  !PREEMPT kernels.  It's very late in the cycle but this definitely is
  a -stable material.

  The problem is that workqueue worker tasks may process unlimited
  number of work items back-to-back without every yielding inbetween.
  This usually isn't noticeable but a work item which re-queues itself
  waiting for someone else to do something can deadlock with
  stop_machine.  stop_machine will ensure nothing else happens on all
  other cpus and the requeueing work item will reqeueue itself
  indefinitely without ever yielding and thus preventing the CPU from
  entering stop_machine.

  Kudos to Jamie Liu for spotting and diagnosing the problem.  This can
  be trivially fixed by adding cond_resched() after processing each work
  item"

* 'for-3.11-fixes' of git://git.kernel.org/pub/scm/linux/kernel/git/tj/wq:
  workqueue: cond_resched() after processing each work item

1 files changed, 9 insertions, 0 deletions

diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index 7f5d4be2203..e93f7b9067d 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -2201,6 +2201,15 @@ __acquires(&pool->lock)
 		dump_stack();
 	}
 
+	/*
+	 * The following prevents a kworker from hogging CPU on !PREEMPT
+	 * kernels, where a requeueing work item waiting for something to
+	 * happen could deadlock with stop_machine as such work item could
+	 * indefinitely requeue itself while all other CPUs are trapped in
+	 * stop_machine.
+	 */
+	cond_resched();
+
 	spin_lock_irq(&pool->lock);
 
 	/* clear cpu intensive status */