[PATCH 42/49] writeback: implement bdi_wait_for_completion()

Tejun Heo
2015-04-06T20:04:14Z

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>
---
fs/fs-writeback.c | 57 +++++++++++++++++++++++++++++++---------
include/linux/backing-dev-defs.h | 2 ++
mm/backing-dev.c | 1 +
3 files changed, 48 insertions(+), 12 deletions(-)

diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
index 306229a..faa9272 100644
--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c
@@ -34,6 +34,10 @@
*/
#define MIN_WRITEBACK_PAGES (4096UL >> (PAGE_CACHE_SHIFT - 10))

+struct wb_completion {
+ atomic_t cnt;
+};
+
/*
* Passed into wb_writeback(), essentially a subset of writeback_control
*/
@@ -51,9 +55,21 @@ struct wb_writeback_work {
enum wb_reason reason; /* why was writeback initiated? */

struct list_head list; /* pending work list */
- struct completion *done; /* set if the caller waits */
+ struct wb_completion *done; /* set if the caller waits */
};

+/*
+ * If one wants to wait for one or more wb_writeback_works, each work's
+ * ->done should be set to a wb_completion defined using the following
+ * macro. Once all work items are issued with wb_queue_work(), the caller
+ * can wait for the completion of all using wb_wait_for_completion(). Work
+ * items which are waited upon aren't freed automatically on completion.
+ */
+#define DEFINE_WB_COMPLETION_ONSTACK(cmpl) \
+ struct wb_completion cmpl = { \
+ .cnt = ATOMIC_INIT(1), \
+ }
+
static inline struct inode *wb_inode(struct list_head *head)
{
return list_entry(head, struct inode, i_wb_list);
@@ -149,17 +165,34 @@ static void wb_queue_work(struct bdi_writeback *wb,
trace_writeback_queue(wb->bdi, work);

spin_lock_bh(&wb->work_lock);
- if (!test_bit(WB_registered, &wb->state)) {
- if (work->done)
- complete(work->done);
+ if (!test_bit(WB_registered, &wb->state))
goto out_unlock;
- }
+ if (work->done)
+ atomic_inc(&work->done->cnt);
list_add_tail(&work->list, &wb->work_list);
mod_delayed_work(bdi_wq, &wb->dwork, 0);
out_unlock:
spin_unlock_bh(&wb->work_lock);
}

+/**
+ * wb_wait_for_completion - wait for completion of bdi_writeback_works
+ * @bdi: bdi work items were issued to
+ * @done: target wb_completion
+ *
+ * Wait for one or more work items issued to @bdi with their ->done field
+ * set to @done, which should have been defined with
+ * DEFINE_WB_COMPLETION_ONSTACK(). This function returns after all such
+ * work items are completed. Work items which are waited upon aren't freed
+ * automatically on completion.
+ */
+static void wb_wait_for_completion(struct backing_dev_info *bdi,
+ struct wb_completion *done)
+{
+ atomic_dec(&done->cnt); /* put down the initial count */
+ wait_event(bdi->wb_waitq, !atomic_read(&done->cnt));
+}
+
#ifdef CONFIG_CGROUP_WRITEBACK

/**
@@ -1125,7 +1158,7 @@ static long wb_do_writeback(struct bdi_writeback *wb)

set_bit(WB_writeback_running, &wb->state);
while ((work = get_next_work_item(wb)) != NULL) {
- struct completion *done = work->done;
+ struct wb_completion *done = work->done;

trace_writeback_exec(wb->bdi, work);

@@ -1133,8 +1166,8 @@ static long wb_do_writeback(struct bdi_writeback *wb)

if (work->auto_free)
kfree(work);
- if (done)
- complete(done);
+ if (done && atomic_dec_and_test(&done->cnt))
+ wake_up_all(&wb->bdi->wb_waitq);
}

/*
@@ -1438,7 +1471,7 @@ void writeback_inodes_sb_nr(struct super_block *sb,
unsigned long nr,
enum wb_reason reason)
{
- DECLARE_COMPLETION_ONSTACK(done);
+ DEFINE_WB_COMPLETION_ONSTACK(done);
struct wb_writeback_work work = {
.sb = sb,
.sync_mode = WB_SYNC_NONE,
@@ -1453,7 +1486,7 @@ void writeback_inodes_sb_nr(struct super_block *sb,
return;
WARN_ON(!rwsem_is_locked(&sb->s_umount));
wb_queue_work(&bdi->wb, &work);
- wait_for_completion(&done);
+ wb_wait_for_completion(bdi, &done);
}
EXPORT_SYMBOL(writeback_inodes_sb_nr);

@@ -1520,7 +1553,7 @@ EXPORT_SYMBOL(try_to_writeback_inodes_sb);
*/
void sync_inodes_sb(struct super_block *sb)
{
- DECLARE_COMPLETION_ONSTACK(done);
+ DEFINE_WB_COMPLETION_ONSTACK(done);
struct wb_writeback_work work = {
.sb = sb,
.sync_mode = WB_SYNC_ALL,
@@ -1538,7 +1571,7 @@ void sync_inodes_sb(struct super_block *sb)
WARN_ON(!rwsem_is_locked(&sb->s_umount));

wb_queue_work(&bdi->wb, &work);
- wait_for_completion(&done);
+ wb_wait_for_completion(bdi, &done);

wait_sb_inodes(sb);
}
diff --git a/include/linux/backing-dev-defs.h b/include/linux/backing-dev-defs.h
index 8c857d7..97a92fa 100644
--- a/include/linux/backing-dev-defs.h
+++ b/include/linux/backing-dev-defs.h
@@ -155,6 +155,8 @@ struct backing_dev_info {
struct rb_root cgwb_congested_tree; /* their congested states */
atomic_t usage_cnt; /* counts both cgwbs and cgwb_contested's */
#endif
+ wait_queue_head_t wb_waitq;
+
struct device *dev;

struct timer_list laptop_mode_wb_timer;
diff --git a/mm/backing-dev.c b/mm/backing-dev.c
index d2f16fc9..ad5608d 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)
--
2.1.0