[patch 09/38] workqueue: fix deadlock in worker_maybe_bind_and_lock()

[Greg KH]

2.6.38-stable review patch.  If anyone has any objections, please let us know.

------------------

From: Tejun Heo <tj@xxxxxxxxxx>

commit 5035b20fa5cd146b66f5f89619c20a4177fb736d upstream.

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@xxxxxxxxxx>
Reported-by: Thilo-Alexander Ginkel <thilo@xxxxxxxxxx>
Tested-by: Thilo-Alexander Ginkel <thilo@xxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxx>

---
 kernel/workqueue.c |    8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -1283,8 +1283,14 @@ __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();
 	}
 }
 


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