On Thu, Sep 03 2009, Christoph Hellwig wrote:
> Clean bdi_start_writeback up a bit:
>
>
> Index: linux-2.6/fs/fs-writeback.c
> ===================================================================
> --- linux-2.6.orig/fs/fs-writeback.c 2009-09-03 23:50:12.880832607 -0300
> +++ linux-2.6/fs/fs-writeback.c 2009-09-03 23:54:42.372832985 -0300
> @@ -70,13 +70,6 @@ static inline void bdi_work_init(struct
> work->state = WS_USED;
> }
>
> -static inline void bdi_work_init_on_stack(struct bdi_work *work,
> - struct writeback_control *wbc)
> -{
> - bdi_work_init(work, wbc);
> - work->state |= WS_ONSTACK;
> -}
> -
> /**
> * writeback_in_progress - determine whether there is writeback in progress
> * @bdi: the device's backing_dev_info structure.
> @@ -207,35 +200,26 @@ static struct bdi_work *bdi_alloc_work(s
>
> void bdi_start_writeback(struct writeback_control *wbc)
> {
> - const bool must_wait = wbc->sync_mode == WB_SYNC_ALL;
> - struct bdi_work work_stack, *work = NULL;
> -
> - if (!must_wait)
> - work = bdi_alloc_work(wbc);
> + struct bdi_work work;
>
> - if (!work) {
> - work = &work_stack;
> - bdi_work_init_on_stack(work, wbc);
> + if (wbc->sync_mode != WB_SYNC_ALL) {
> + struct bdi_work *w = bdi_alloc_work(wbc);
> + if (w) {
> + bdi_queue_work(wbc->bdi, w);
> + return;
> + }
> }
>
> - bdi_queue_work(wbc->bdi, work);
> + bdi_work_init(&work, wbc);
> + work.state |= WS_ONSTACK;
> + bdi_queue_work(wbc->bdi, &work);
>
> /*
> * If the sync mode is WB_SYNC_ALL, block waiting for the work to
> - * complete. If not, we only need to wait for the work to be started,
> - * if we allocated it on-stack. We use the same mechanism, if the
> - * wait bit is set in the bdi_work struct, then threads will not
> - * clear pending until after they are done.
> - *
> - * Note that work == &work_stack if must_wait is true, so we don't
> - * need to do call_rcu() here ever, since the completion path will
> - * have done that for us.
> + * complete.
> */
> - if (must_wait || work == &work_stack) {
> - bdi_wait_on_work_clear(work);
> - if (work != &work_stack)
> - call_rcu(&work->rcu_head, bdi_work_free);
> - }
> + if (wbc->sync_mode == WB_SYNC_ALL)
> + bdi_wait_on_work_clear(&work);
> }
That doesn't work, you have to wait for on-stack work. So either we just
punt and not do anything for WB_SYNC_NONE if the allocation fails, or we
punt to stack and do the wait. Since it's a cleaning action and
allocation fails, falling back to the stack and waiting seems like the
most appropriate choice.
--
Jens Axboe
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