workqueue: Add quiescent state between work items

Similar to the stop_machine deadlock scenario on !PREEMPT kernels
addressed in b22ce278 "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.
Signed-off-by: NJoe Lawrence <joe.lawrence@stratus.com>
Link: https://lkml.kernel.org/r/20140926105227.01325697@jlaw-desktop.mno.stratus.com
Link: https://lkml.kernel.org/r/20140929115445.40221d8e@jlaw-desktop.mno.stratus.com
Fixes: b22ce278 ("workqueue: cond_resched() after processing each work item")
Cc: <stable@vger.kernel.org>
Acked-by: NTejun Heo <tj@kernel.org>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

kernel/workqueue.c

@@ -2043,8 +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.
 	 */
+	rcu_note_voluntary_context_switch(current);
 	cond_resched();
 
 	spin_lock_irq(&pool->lock);