Re: [PATCH 1/2] workqueue: skip lockdep wq dependency in cancel_work_sync()

Tejun Heo
2018-08-21T16:08:21Z

On Tue, Aug 21, 2018 at 02:03:16PM +0200, Johannes Berg wrote:
> From: Johannes Berg <johannes.berg@intel.com>
>
> In cancel_work_sync(), we can only have one of two cases, even
> with an ordered workqueue:
> * the work isn't running, just cancelled before it started
> * the work is running, but then nothing else can be on the
> workqueue before it
>
> Thus, we need to skip the lockdep workqueue dependency handling,
> otherwise we get false positive reports from lockdep saying that
> we have a potential deadlock when the workqueue also has other
> work items with locking, e.g.
>
> work1_function() { mutex_lock(&mutex); ... }
> work2_function() { /* nothing */ }
>
> other_function() {
> queue_work(ordered_wq, &work1);
> queue_work(ordered_wq, &work2);
> mutex_lock(&mutex);
> cancel_work_sync(&work2);
> }
>
> As described above, this isn't a problem, but lockdep will
> currently flag it as if cancel_work_sync() was flush_work(),
> which *is* a problem.
>
> Signed-off-by: Johannes Berg <johannes.berg@intel.com>
> ---
> kernel/workqueue.c | 37 ++++++++++++++++++++++---------------
> 1 file changed, 22 insertions(+), 15 deletions(-)
>
> diff --git a/kernel/workqueue.c b/kernel/workqueue.c
> index 78b192071ef7..a6c2b823f348 100644
> --- a/kernel/workqueue.c
> +++ b/kernel/workqueue.c
> @@ -2843,7 +2843,8 @@ void drain_workqueue(struct workqueue_struct *wq)
> }
> EXPORT_SYMBOL_GPL(drain_workqueue);
>
> -static bool start_flush_work(struct work_struct *work, struct wq_barrier *barr)
> +static bool start_flush_work(struct work_struct *work, struct wq_barrier *barr,
> + bool from_cancel)
> {
> struct worker *worker = NULL;
> struct worker_pool *pool;
> @@ -2885,7 +2886,8 @@ static bool start_flush_work(struct work_struct *work, struct wq_barrier *barr)
> * workqueues the deadlock happens when the rescuer stalls, blocking
> * forward progress.
> */
> - if (pwq->wq->saved_max_active == 1 || pwq->wq->rescuer) {
> + if (!from_cancel &&
> + (pwq->wq->saved_max_active == 1 || pwq->wq->rescuer)) {
> lock_map_acquire(&pwq->wq->lockdep_map);
> lock_map_release(&pwq->wq->lockdep_map);
> }

But this can lead to a deadlock. I'd much rather err on the side of
discouraging complex lock dancing around ordered workqueues, no?

Thanks.

--
tejun