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Subject[PATCH] workqueue: fix deadlock in worker_maybe_bind_and_lock()
From 5035b20fa5cd146b66f5f89619c20a4177fb736d Mon Sep 17 00:00:00 2001
From: Tejun Heo <>
Date: Fri, 29 Apr 2011 18:08:37 +0200

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.

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 <>
Reported-by: Thilo-Alexander Ginkel <>
Tested-by: Thilo-Alexander Ginkel <>
Will soon send pull request to Linus. Thank you very much.

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

diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index 04ef830..e3378e8 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -1291,8 +1291,14 @@ __acquires(&gcwq->lock)
return true;

- /* 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.
+ */
+ cond_resched();


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