Tetsuo Handa wrote: > Jan Kara wrote: > > Make wb_workfn() use wakeup_wb() for requeueing the work which takes all > > the necessary precautions against racing with bdi unregistration. > > Yes, this patch will solve NULL pointer dereference bug. But is it OK to leave > list_empty(&wb->work_list) == false situation? Who takes over the role of making > list_empty(&wb->work_list) == true? syzbot is again reporting the same NULL pointer dereference. general protection fault in wb_workfn (2) https://syzkaller.appspot.com/bug?id=e0818ccb7e46190b3f1038b0c794299208ed4206 Didn't we overlook something obvious in commit b8b784958eccbf8f ("bdi: Fix oops in wb_workfn()") ? At first, I thought that that commit will solve NULL pointer dereference bug. But what does if (!list_empty(&wb->work_list)) - mod_delayed_work(bdi_wq, &wb->dwork, 0); + wb_wakeup(wb); else if (wb_has_dirty_io(wb) && dirty_writeback_interval) wb_wakeup_delayed(wb); mean? static void wb_wakeup(struct bdi_writeback *wb) { spin_lock_bh(&wb->work_lock); if (test_bit(WB_registered, &wb->state)) mod_delayed_work(bdi_wq, &wb->dwork, 0); spin_unlock_bh(&wb->work_lock); } It means nothing but "we don't call mod_delayed_work() if WB_registered bit was already cleared". But if WB_registered bit is not yet cleared when we hit wb_wakeup_delayed() path? void wb_wakeup_delayed(struct bdi_writeback *wb) { unsigned long timeout; timeout = msecs_to_jiffies(dirty_writeback_interval * 10); spin_lock_bh(&wb->work_lock); if (test_bit(WB_registered, &wb->state)) queue_delayed_work(bdi_wq, &wb->dwork, timeout); spin_unlock_bh(&wb->work_lock); } add_timer() is called because (presumably) timeout > 0. And after that timeout expires, __queue_work() is called even if WB_registered bit is already cleared before that timeout expires, isn't it? 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? Then, don't we need to check that mod_delayed_work(bdi_wq, &wb->dwork, 0); flush_delayed_work(&wb->dwork); is really waiting for completion? At least, shouldn't we try below debug output (not only for debugging this report but also generally desirable)? diff --git a/mm/backing-dev.c b/mm/backing-dev.c index 7441bd9..ccec8cd 100644 --- a/mm/backing-dev.c +++ b/mm/backing-dev.c @@ -376,8 +376,10 @@ static void wb_shutdown(struct bdi_writeback *wb) * tells wb_workfn() that @wb is dying and its work_list needs to * be drained no matter what. */ - mod_delayed_work(bdi_wq, &wb->dwork, 0); - flush_delayed_work(&wb->dwork); + if (!mod_delayed_work(bdi_wq, &wb->dwork, 0)) + printk(KERN_WARNING "wb_shutdown: mod_delayed_work() failed\n"); + if (!flush_delayed_work(&wb->dwork)) + printk(KERN_WARNING "wb_shutdown: flush_delayed_work() failed\n"); WARN_ON(!list_empty(&wb->work_list)); /* * Make sure bit gets cleared after shutdown is finished. Matches with
