workqueue: Release PENDING in __queue_work() drain/destroy reject path

The caller of __queue_work() owns WORK_STRUCT_PENDING, won via test_and_set_bit() in queue_work_on()/__queue_delayed_work(). The state machine documented above __queue_work() requires that owner to either hand the token to a pwq (insert_work() -> set_work_pwq()), hand it to a timer, or release it via set_work_pool_and_clear_pending(). try_to_grab_pending() relies on this: when it observes "PENDING && off-queue" it busy-loops, trusting the current owner to make progress. The (__WQ_DESTROYING | __WQ_DRAINING) early-return path violates that contract. It WARN_ONCE()s and bare-returns, leaving work->data with PENDING set, WORK_STRUCT_PWQ clear, and work->entry empty. The path is reachable without explicit API abuse: queue_delayed_work() arms a timer with PENDING set; if drain_workqueue() runs while the timer is still pending, delayed_work_timer_fn() -> __queue_work() in softirq context hits the WARN, current is not a wq worker so is_chained_work() is false, and the work is silently dropped with PENDING leaked. Mirror what clear_pending_if_disabled() already does on its analogous reject path: unpack the off-queue data and call set_work_pool_and_clear_pending() to release the token before returning. I was able to reproduce this by queueing several slow works on a max_active=1 wq, arm a delayed_work whose timer fires while drain_workqueue() is blocked, then call cancel_delayed_work_sync(). Without this patch the cancel livelocks at 100% CPU; with it the cancel returns immediately.

BLBreno Leitao committed 6 months ago