Hello, Linus.
This pull request 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.
Thanks.
The following changes since commit c95389b4cd6a4b52af78bea706a274453e886251:
Merge branch 'akpm' (patches from Andrew Morton) (2013-08-28 19:31:33 -0700)
are available in the git repository at:
git://git.kernel.org/pub/scm/linux/kernel/git/tj/wq.git for-3.11-fixes
for you to fetch changes up to b22ce2785d97423846206cceec4efee0c4afd980:
workqueue: cond_resched() after processing each work item (2013-08-29
09:19:28 -0400)
----------------------------------------------------------------
Tejun Heo (1):
workqueue: cond_resched() after processing each work item
kernel/workqueue.c | 9 +++++++++
1 file changed, 9 insertions(+)
diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index 7f5d4be..e93f7b9 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 */
--
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