Jan Kara wrote: > > void delayed_work_timer_fn(struct timer_list *t) > > { > > struct delayed_work *dwork = from_timer(dwork, t, timer); > > > > /* should have been called from irqsafe timer with irq already off */ > > __queue_work(dwork->cpu, dwork->wq, &dwork->work); > > } > > > > Then, wb_workfn() is after all scheduled even if we check for > > WB_registered bit, isn't it? > > It can be queued after WB_registered bit is cleared but it cannot be queued > after mod_delayed_work(bdi_wq, &wb->dwork, 0) has finished. That function > deletes the pending timer (the timer cannot be armed again because > WB_registered is cleared) and queues what should be the last round of > wb_workfn(). mod_delayed_work() deletes the pending timer but does not wait for already invoked timer handler to complete because it is using del_timer() rather than del_timer_sync(). Then, what happens if __queue_work() is almost concurrently executed from two CPUs, one from mod_delayed_work(bdi_wq, &wb->dwork, 0) from wb_shutdown() path (which is called without spin_lock_bh(&wb->work_lock)) and the other from delayed_work_timer_fn() path (which is called without checking WB_registered bit under spin_lock_bh(&wb->work_lock)) ? wb_wakeup_delayed() { spin_lock_bh(&wb->work_lock); if (test_bit(WB_registered, &wb->state)) // succeeds queue_delayed_work(bdi_wq, &wb->d_work, timeout) { queue_delayed_work_on(WORK_CPU_UNBOUND, bdi_wq, &wb->d_work, timeout) { if (!test_and_set_bit(WORK_STRUCT_PENDING_BIT, work_data_bits(&wb->d_work.work))) { // succeeds __queue_delayed_work(WORK_CPU_UNBOUND, bdi_wq, &wb->d_work, timeout) { add_timer(timer); // schedules for delayed_work_timer_fn() } } } } spin_unlock_bh(&wb->work_lock); } delayed_work_timer_fn() { // del_timer() already returns false at this point because this timer // is already inside handler. But something took long here enough to // wait for __queue_work() from wb_shutdown() path to finish? __queue_work(WORK_CPU_UNBOUND, bdi_wq, &wb->d_work.work) { insert_work(pwq, work, worklist, work_flags); } } wb_shutdown() { mod_delayed_work(bdi_wq, &wb->dwork, 0) { mod_delayed_work_on(WORK_CPU_UNBOUND, bdi_wq, &wb->dwork, 0) { ret = try_to_grab_pending(&wb->dwork.work, true, &flags) { if (likely(del_timer(&wb->dwork.timer))) // fails because already in delayed_work_timer_fn() return 1; if (!test_and_set_bit(WORK_STRUCT_PENDING_BIT, work_data_bits(&wb->dwork.work))) // fails because already set by queue_delayed_work() return 0; // Returns 1 or -ENOENT after doing something? } if (ret >= 0) __queue_delayed_work(WORK_CPU_UNBOUND, bdi_wq, &wb->dwork, 0) { __queue_work(WORK_CPU_UNBOUND, bdi_wq, &wb->dwork.work) { insert_work(pwq, work, worklist, work_flags); } } } } }