This is a note to let you know that I've just added the patch titled
workqueue: cond_resched() after processing each work item
to the 3.4-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary
The filename of the patch is:
workqueue-cond_resched-after-processing-each-work-item.patch
and it can be found in the queue-3.4 subdirectory.
If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.
>From fbbd6511ab0dff8a79fc5803250b77a1260be354 Mon Sep 17 00:00:00 2001
From: Tejun Heo <tj@xxxxxxxxxx>
Date: Wed, 28 Aug 2013 17:33:37 -0400
Subject: workqueue: cond_resched() after processing each work item
From: Tejun Heo <tj@xxxxxxxxxx>
commit b22ce2785d97423846206cceec4efee0c4afd980 upstream.
If !PREEMPT, a kworker running work items back to back can hog CPU.
This becomes dangerous when a self-requeueing work item which is
waiting for something to happen races against stop_machine. Such
self-requeueing work item would requeue itself indefinitely hogging
the kworker and CPU it's running on while stop_machine would wait for
that CPU to enter stop_machine while preventing anything else from
happening on all other CPUs. The two would deadlock.
Jamie Liu reports that this deadlock scenario exists around
scsi_requeue_run_queue() and libata port multiplier support, where one
port may exclude command processing from other ports. With the right
timing, scsi_requeue_run_queue() can end up requeueing itself trying
to execute an IO which is asked to be retried while another device has
an exclusive access, which in turn can't make forward progress due to
stop_machine.
Fix it by invoking cond_resched() after executing each work item.
Signed-off-by: Tejun Heo <tj@xxxxxxxxxx>
Reported-by: Jamie Liu <jamieliu@xxxxxxxxxx>
References: http://thread.gmane.org/gmane.linux.kernel/1552567
[bwh: Backported to 3.2: adjust context]
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
Cc: Qiang Huang <h.huangqiang@xxxxxxxxxx>
Cc: Li Zefan <lizefan@xxxxxxxxxx>
Cc: Jianguo Wu <wujianguo@xxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
kernel/workqueue.c | 9 +++++++++
1 file changed, 9 insertions(+)
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -1922,6 +1922,15 @@ __acquires(&gcwq->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(&gcwq->lock);
/* clear cpu intensive status */
Patches currently in stable-queue which might be from tj@xxxxxxxxxx are
queue-3.4/workqueue-cond_resched-after-processing-each-work-item.patch
queue-3.4/idr-fix-top-layer-handling.patch
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