workqueue: fix spurious CPU locality WARN from process_one_work()

25511a4776 "workqueue: reimplement CPU online rebinding to handle idle
workers" added CPU locality sanity check in process_one_work().  It
triggers if a worker is executing on a different CPU without UNBOUND
or REBIND set.

This works for all normal workers but rescuers can trigger this
spuriously when they're serving the unbound or a disassociated
global_cwq - rescuers don't have either flag set and thus its
gcwq->cpu can be a different value including %WORK_CPU_UNBOUND.

Fix it by additionally testing %GCWQ_DISASSOCIATED.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reported-by: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
LKML-Refence: <20120721213656.GA7783@linux.vnet.ibm.com>

kernel/workqueue.c

@@ -1949,7 +1949,13 @@ __acquires(&gcwq->lock)
 
 	lockdep_copy_map(&lockdep_map, &work->lockdep_map);
 #endif
+	/*
+	 * Ensure we're on the correct CPU.  DISASSOCIATED test is
+	 * necessary to avoid spurious warnings from rescuers servicing the
+	 * unbound or a disassociated gcwq.
+	 */
 	WARN_ON_ONCE(!(worker->flags & (WORKER_UNBOUND | WORKER_REBIND)) &&
+		     !(gcwq->flags & GCWQ_DISASSOCIATED) &&
 		     raw_smp_processor_id() != gcwq->cpu);
 
 	/*