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

Tetsuo Handa
2018-06-13T11:51:13Z

Tetsuo Handa wrote:
> Or, toggling a dedicated flag using test_and_change_bit():
>
>
> include/linux/backing-dev-defs.h | 1 +
> mm/backing-dev.c | 8 +++++++-
> 2 files changed, 8 insertions(+), 1 deletion(-)
>
> diff --git a/include/linux/backing-dev-defs.h b/include/linux/backing-dev-defs.h
> index 0bd432a..93ff83c 100644
> --- a/include/linux/backing-dev-defs.h
> +++ b/include/linux/backing-dev-defs.h
> @@ -26,6 +26,7 @@ enum wb_state {
> WB_writeback_running, /* Writeback is in progress */
> WB_has_dirty_io, /* Dirty inodes on ->b_{dirty|io|more_io} */
> WB_start_all, /* nr_pages == 0 (all) work pending */
> + WB_postpone_kfree, /* cgwb_bdi_unregister() will access later */
> };
>
> enum wb_congested_state {
> diff --git a/mm/backing-dev.c b/mm/backing-dev.c
> index 347cc83..422d7a7 100644
> --- a/mm/backing-dev.c
> +++ b/mm/backing-dev.c
> @@ -516,7 +516,10 @@ static void cgwb_release_workfn(struct work_struct *work)
> fprop_local_destroy_percpu(&wb->memcg_completions);
> percpu_ref_exit(&wb->refcnt);
> wb_exit(wb);
> - kfree_rcu(wb, rcu);
> + spin_lock_irq(&cgwb_lock);
> + if (!test_and_change_bit(WB_postpone_kfree, &wb->state))
> + kfree_rcu(wb, rcu);
> + spin_unlock_irq(&cgwb_lock);
> }
>
> static void cgwb_release(struct percpu_ref *refcnt)
> @@ -721,9 +724,12 @@ static void cgwb_bdi_unregister(struct backing_dev_info *bdi)
> while (!list_empty(&bdi->wb_list)) {
> wb = list_first_entry(&bdi->wb_list, struct bdi_writeback,
> bdi_node);
> + set_bit(WB_postpone_kfree, &wb->state);
> spin_unlock_irq(&cgwb_lock);
> wb_shutdown(wb);
> spin_lock_irq(&cgwb_lock);
> + if (!test_and_change_bit(WB_postpone_kfree, &wb->state))
> + kfree_rcu(wb, rcu);
> }
> spin_unlock_irq(&cgwb_lock);
> }

Forgot to include below change, but isn't this approach the simplest?

@@ -370,7 +370,6 @@ static void wb_shutdown(struct bdi_writeback *wb)
set_bit(WB_shutting_down, &wb->state);
spin_unlock_bh(&wb->work_lock);

- cgwb_remove_from_bdi_list(wb);
/*
* Drain work list and shutdown the delayed_work. !WB_registered
* tells wb_workfn() that @wb is dying and its work_list needs to
@@ -379,6 +378,7 @@ static void wb_shutdown(struct bdi_writeback *wb)
mod_delayed_work(bdi_wq, &wb->dwork, 0);
flush_delayed_work(&wb->dwork);
WARN_ON(!list_empty(&wb->work_list));
+ cgwb_remove_from_bdi_list(wb);
/*
* Make sure bit gets cleared after shutdown is finished. Matches with
* the barrier provided by test_and_clear_bit() above.
--