Commit 5035b20f authored Apr 29, 2011 by Tejun Heo

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 <tj@kernel.org>
Reported-by: Thilo-Alexander Ginkel <thilo@ginkel.com>
Tested-by: Thilo-Alexander Ginkel <thilo@ginkel.com>
Cc: stable@kernel.org

parent e2de9e08

kernel/workqueue.c

+7 −1

Original line number	Diff line number	Diff line
		@@ -1291,8 +1291,14 @@ __acquires(&gcwq->lock)
		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();
		}
		}