[PATCH] workqueue: add quiescent state between work items

[Joe Lawrence]

Similar to the stop_machine deadlock scenario on !PREEMPT kernels
addressed in b22ce2785d97 "workqueue: cond_resched() after processing
each work item", kworker threads requeueing back-to-back with zero jiffy
delay can stall RCU. The cond_resched call introduced in that fix will
yield only iff there are other higher priority tasks to run, so force a
quiescent RCU state between work items with cond_resched_rcu_qs.

Signed-off-by: Joe Lawrence <joe.lawrence@xxxxxxxxxxx>
Link: https://lkml.kernel.org/r/20140926105227.01325697@xxxxxxxxxxxxxxxxxxxxxxxxxxxx
Link: https://lkml.kernel.org/r/20140929115445.40221d8e@xxxxxxxxxxxxxxxxxxxxxxxxxxxx
---
 kernel/workqueue.c |    5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index 5dbe22a..09b685d 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -2043,9 +2043,10 @@ __acquires(&pool->lock)
 	 * 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.
+	 * stop_machine. At the same time, report a quiescent RCU state so
+	 * the same condition doesn't freeze RCU.
 	 */
-	cond_resched();
+	cond_resched_rcu_qs();
 
 	spin_lock_irq(&pool->lock);
 
-- 
1.7.10.4

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