The scheduler currently has different queues for starting jobs and cancellation jobs to sync with main thread and worker thread. It should be handle sequential for registering the starting job and canceling the job.
Created attachment 325592 [details] patch
Comment on attachment 325592 [details] patch Clearing flags on attachment: 325592 Committed r224322: <https://trac.webkit.org/changeset/224322>
All reviewed patches have been landed. Closing bug.
<rdar://problem/35567527>