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SubjectRe: [PATCH 1/2] workqueue: Catch more locking problems with flush_work()
On Wed, Apr 18, 2012 at 08:25:57PM -0700, Stephen Boyd wrote:
> If a workqueue is flushed but the work item is not scheduled to
> run, lockdep checking will be circumvented. For example:
>
> static DEFINE_MUTEX(mutex);
>
> static void my_work(struct work_struct *w)
> {
> mutex_lock(&mutex);
> mutex_unlock(&mutex);
> }
>
> static DECLARE_WORK(work, my_work);
>
> static int __init start_test_module(void)
> {
> schedule_work(&work);
> return 0;
> }
> module_init(start_test_module);
>
> static void __exit stop_test_module(void)
> {
> mutex_lock(&mutex);
> flush_work(&work);
> mutex_unlock(&mutex);
> }
> module_exit(stop_test_module);
>
> would only print a warning if the work item was actively running
> when flush_work() was called. Otherwise flush_work() returns
> early. In this trivial example nothing could go wrong, but if the
> work item is schedule via an interrupt we could potentially have a
> scenario where the work item is running just at the time flush_work()

You mean flush_work() could be called in interupt? I don't it is
possible.

> is called. This could become a classic AB-BA locking problem.

I don't see how the deadlock happen, could you please be more
specific?

Thanks,
Yong


>
> Add a lockdep hint in flush_work() in the "work not running" case
> so that we always catch this potential deadlock scenario.
>
> Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>
> ---
> kernel/workqueue.c | 5 ++++-
> 1 file changed, 4 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/workqueue.c b/kernel/workqueue.c
> index 66ec08d..eb800df 100644
> --- a/kernel/workqueue.c
> +++ b/kernel/workqueue.c
> @@ -2513,8 +2513,11 @@ bool flush_work(struct work_struct *work)
> wait_for_completion(&barr.done);
> destroy_work_on_stack(&barr.work);
> return true;
> - } else
> + } else {
> + lock_map_acquire(&work->lockdep_map);
> + lock_map_release(&work->lockdep_map);
> return false;
> + }
> }
> EXPORT_SYMBOL_GPL(flush_work);
>
> --
> Sent by an employee of the Qualcomm Innovation Center, Inc.
> The Qualcomm Innovation Center, Inc. is a member of the Code Aurora Forum.
>
> --
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