Re: [PATCH 15/49] writeback: move backing_dev_info->wb_lock and ->worklist into bdi_writeback

Jan Kara
2015-04-20T15:32:35Z

On Mon 06-04-15 15:58:04, Tejun Heo wrote:
> Currently, a bdi (backing_dev_info) embeds single wb (bdi_writeback)
> and the role of the separation is unclear. For cgroup support for
> writeback IOs, a bdi will be updated to host multiple wb's where each
> wb serves writeback IOs of a different cgroup on the bdi. To achieve
> that, a wb should carry all states necessary for servicing writeback
> IOs for a cgroup independently.
>
> This patch moves bdi->wb_lock and ->worklist into wb.
>
> * The lock protects bdi->worklist and bdi->wb.dwork scheduling. While
> moving, rename it to wb->work_lock as wb->wb_lock is confusing.
> Also, move wb->dwork downwards so that it's colocated with the new
> ->work_lock and ->work_list fields.
>
> * bdi_writeback_workfn() -> wb_workfn()
> bdi_wakeup_thread_delayed(bdi) -> wb_wakeup_delayed(wb)
> bdi_wakeup_thread(bdi) -> wb_wakeup(wb)
> bdi_queue_work(bdi, ...) -> wb_queue_work(wb, ...)
> __bdi_start_writeback(bdi, ...) -> __wb_start_writeback(wb, ...)
> get_next_work_item(bdi) -> get_next_work_item(wb)
>
> * bdi_wb_shutdown() is renamed to wb_shutdown() and now takes @wb.
> The function contained parts which belong to the containing bdi
> rather than the wb itself - testing cap_writeback_dirty and
> bdi_remove_from_list() invocation. Those are moved to
> bdi_unregister().
>
> * bdi_wb_{init|exit}() are renamed to wb_{init|exit}().
> Initializations of the moved bdi->wb_lock and ->work_list are
> relocated from bdi_init() to wb_init().
>
> * As there's still only one bdi_writeback per backing_dev_info, all
> uses of bdi->state are mechanically replaced with bdi->wb.state
> introducing no behavior changes.
>
...
> @@ -454,9 +451,9 @@ EXPORT_SYMBOL(bdi_init);
>
> void bdi_destroy(struct backing_dev_info *bdi)
> {
> - bdi_wb_shutdown(bdi);
> -
> - WARN_ON(!list_empty(&bdi->work_list));
> + /* make sure nobody finds us on the bdi_list anymore */
> + bdi_remove_from_list(bdi);
> + wb_shutdown(&bdi->wb);
>
> if (bdi->dev) {
> bdi_debug_unregister(bdi);
But if someone ends up calling bdi_destroy() on unregistered bdi,
bdi_remove_from_list() will be corrupting memory, won't it? And if I
remember right there were some corner cases where this really happened.
Previously we were careful and checked WB_registered. I guess we could
check for !list_empty(&bdi->bdi_list) and also reinit bdi_list in
bdi_remove_from_list() after synchronize_rcu_expedited().

Honza

--
Jan Kara <jack@suse.cz>
SUSE Labs, CR