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>

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 +55,22 @@ 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),			\
+	}
+
+
 /*
  * If an inode is constantly having its pages dirtied, but then the
  * updates stop dirtytime_expire_interval seconds in the past, it's
@@ -161,17 +178,34 @@ 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
 
 /**
@@ -1143,7 +1177,7 @@ 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);
 
@@ -1151,8 +1185,8 @@ 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);
 	}
 
 	/*
@@ -1518,7 +1552,7 @@ 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,
@@ -1533,7 +1567,7 @@ 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);
 
@@ -1600,7 +1634,7 @@ 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,
@@ -1618,7 +1652,7 @@ 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);
 }

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;

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)