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

1 files changed, 7 insertions, 1 deletions

diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index 04ef830690ec..e3378e8d3a5c 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -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();
         }
 }