Re: [PATCH] bdi: Fix oops in wb_workfn()

[Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>]

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