Re: INFO: task hung in wb_shutdown (2)

[Linus Torvalds]

On Tue, May 1, 2018 at 3:27 AM Tetsuo Handa <
penguin-kernel@xxxxxxxxxxxxxxxxxxx> wrote:

> Can you review this patch? syzbot has hit this bug for nearly 4000 times
but
> is still unable to find a reproducer. Therefore, the only way to test
would be
> to apply this patch upstream and test whether the problem is solved.

Looks ok to me, except:

> >       smp_wmb();
> >       clear_bit(WB_shutting_down, &wb->state);
> > +     smp_mb(); /* advised by wake_up_bit() */
> > +     wake_up_bit(&wb->state, WB_shutting_down);

This whole sequence really should just be a pattern with a helper function.

And honestly, the pattern probably *should* be

     clear_bit_unlock(bit, &mem);
     smp_mb__after_atomic()
     wake_up_bit(&mem, bit);

which looks like it is a bit cleaner wrt memory ordering rules.

             Linus