workqueue: fix deadlock in worker_maybe_bind_and_lock()

If a rescuer and stop_machine() bringing down a CPU race with each
other, they may deadlock on non-preemptive kernel. The CPU won't
accept a new task, so the rescuer can't migrate to the target CPU,
while stop_machine() can't proceed because the rescuer is holding one
of the CPU retrying migration. GCWQ_DISASSOCIATED is never cleared
and worker_maybe_bind_and_lock() retries indefinitely.

This problem can be reproduced semi reliably while the system is
entering suspend.

http://thread.gmane.org/gmane.linux.kernel/1122051

A lot of kudos to Thilo-Alexander for reporting this tricky issue and
painstaking testing.

stable: This affects all kernels with cmwq, so all kernels since and
including v2.6.36 need this fix.

Signed-off-by: Tejun Heo <[e-mail]>
Reported-by: Thilo-Alexander Ginkel <[e-mail]>
Tested-by: Thilo-Alexander Ginkel <[e-mail]>
Cc: [e-mail]

kernel/workqueue.c

···
 			return true;
 		spin_unlock_irq(&gcwq->lock);
 
-		/* CPU has come up inbetween, retry migration */
+		/*
+		 * We've raced with CPU hot[un]plug.  Give it a breather
+		 * and retry migration.  cond_resched() is required here;
+		 * otherwise, we might deadlock against cpu_stop trying to
+		 * bring down the CPU on non-preemptive kernel.
+		 */
 		cpu_relax();
+		cond_resched();
 	}
 }