workqueue: fix deadlock in worker_maybe_bind_and_lock()
On Friday, April 29, 2011, Tejun Heo wrote:
> From 5035b20fa5cd146b66f5f89619c20a4177fb736d Mon Sep 17 00:00:00 2001
> From: Tejun Heo <firstname.lastname@example.org>
> 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.
Well, _that_ explains quite a number of mysterious reports where
suspend or poweroff hang randomly.
Thanks a lot of fixing it!
> Signed-off-by: Tejun Heo <email@example.com>
> Reported-by: Thilo-Alexander Ginkel <firstname.lastname@example.org>
> Tested-by: Thilo-Alexander Ginkel <email@example.com>
> Cc: firstname.lastname@example.org
> 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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