writeback: implement bdi_wait_for_completion()

If the completion of a wb_writeback_work can be waited upon by setting
its ->done to a struct completion and waiting on it; however, for
cgroup writeback support, it's necessary to issue multiple work items
to multiple bdi_writebacks and wait for the completion of all.

This patch implements wb_completion which can wait for multiple work
items and replaces the struct completion with it.  It can be defined
using DEFINE_WB_COMPLETION_ONSTACK(), used for multiple work items and
waited for by wb_wait_for_completion().

Nobody currently issues multiple work items and this patch doesn't
introduce any behavior changes.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Jan Kara <jack@suse.cz>
Signed-off-by: Jens Axboe <axboe@fb.com>

1 files changed, 1 insertions, 0 deletions

diff --git a/mm/backing-dev.c b/mm/backing-dev.c
index d2f16fc9069a..ad5608d01e8c 100644
--- a/mm/backing-dev.c
+++ b/mm/backing-dev.c
@@ -768,6 +768,7 @@ int bdi_init(struct backing_dev_info *bdi)
 	bdi->max_ratio = 100;
 	bdi->max_prop_frac = FPROP_FRAC_BASE;
 	INIT_LIST_HEAD(&bdi->bdi_list);
+	init_waitqueue_head(&bdi->wb_waitq);
 
 	err = wb_init(&bdi->wb, bdi, GFP_KERNEL);
 	if (err)