writeback: simplify the write back thread queue

Index: linux-2.6/fs/fs-writeback.c
===================================================================
@@ -38,43 +38,18 @@ int nr_pdflush_threads;
 /*
  * Passed into wb_writeback(), essentially a subset of writeback_control
  */
-struct wb_writeback_args {
+struct wb_writeback_work {
 	long nr_pages;
 	struct super_block *sb;
 	enum writeback_sync_modes sync_mode;
 	unsigned int for_kupdate:1;
 	unsigned int range_cyclic:1;
 	unsigned int for_background:1;
-};
 
-/*
- * Work items for the bdi_writeback threads
- */
-struct bdi_work {
 	struct list_head list;		/* pending work list */
-	struct rcu_head rcu_head;	/* for RCU free/clear of work */
-
-	unsigned long seen;		/* threads that have seen this work */
-	atomic_t pending;		/* number of threads still to do work */
-
-	struct wb_writeback_args args;	/* writeback arguments */
-
-	unsigned long state;		/* flag bits, see WS_* */
-};
-
-enum {
-	WS_INPROGRESS = 0,
-	WS_ONSTACK,
+	struct completion *done;	/* set if the caller waits */
 };
 
-static inline void bdi_work_init(struct bdi_work *work,
-				 struct wb_writeback_args *args)
-{
-	INIT_RCU_HEAD(&work->rcu_head);
-	work->args = *args;
-	__set_bit(WS_INPROGRESS, &work->state);
-}
-
 /**
  * writeback_in_progress - determine whether there is writeback in progress
  * @bdi: the device's backing_dev_info structure.

@@ -87,49 +62,11 @@ int writeback_in_progress(struct backing
 	return !list_empty(&bdi->work_list);
 }
 
-static void bdi_work_free(struct rcu_head *head)
+static void bdi_queue_work(struct backing_dev_info *bdi,
+		struct wb_writeback_work *work)
 {
-	struct bdi_work *work = container_of(head, struct bdi_work, rcu_head);
-
-	clear_bit(WS_INPROGRESS, &work->state);
-	smp_mb__after_clear_bit();
-	wake_up_bit(&work->state, WS_INPROGRESS);
-
-	if (!test_bit(WS_ONSTACK, &work->state))
-		kfree(work);
-}
-
-static void wb_clear_pending(struct bdi_writeback *wb, struct bdi_work *work)
-{
-	/*
-	 * The caller has retrieved the work arguments from this work,
-	 * drop our reference. If this is the last ref, delete and free it
-	 */
-	if (atomic_dec_and_test(&work->pending)) {
-		struct backing_dev_info *bdi = wb->bdi;
-
-		spin_lock(&bdi->wb_lock);
-		list_del_rcu(&work->list);
-		spin_unlock(&bdi->wb_lock);
-
-		call_rcu(&work->rcu_head, bdi_work_free);
-	}
-}
-
-static void bdi_queue_work(struct backing_dev_info *bdi, struct bdi_work *work)
-{
-	work->seen = bdi->wb_mask;
-	BUG_ON(!work->seen);
-	atomic_set(&work->pending, bdi->wb_cnt);
-	BUG_ON(!bdi->wb_cnt);
-
-	/*
-	 * list_add_tail_rcu() contains the necessary barriers to
-	 * make sure the above stores are seen before the item is
-	 * noticed on the list
-	 */
 	spin_lock(&bdi->wb_lock);
-	list_add_tail_rcu(&work->list, &bdi->work_list);
+	list_add_tail(&work->list, &bdi->work_list);
 	spin_unlock(&bdi->wb_lock);
 
 	/*

@@ -146,55 +83,29 @@ static void bdi_queue_work(struct backin
 	}
 }
 
-/*
- * Used for on-stack allocated work items. The caller needs to wait until
- * the wb threads have acked the work before it's safe to continue.
- */
-static void bdi_wait_on_work_done(struct bdi_work *work)
-{
-	wait_on_bit(&work->state, WS_INPROGRESS, bdi_sched_wait,
-		    TASK_UNINTERRUPTIBLE);
-}
-
-static void bdi_alloc_queue_work(struct backing_dev_info *bdi,
-				 struct wb_writeback_args *args)
+static void
+__bdi_start_writeback(struct backing_dev_info *bdi, long nr_pages,
+		bool range_cyclic, bool for_background)
 {
-	struct bdi_work *work;
+	struct wb_writeback_work *work;
 
 	/*
 	 * This is WB_SYNC_NONE writeback, so if allocation fails just
 	 * wakeup the thread for old dirty data writeback
 	 */
-	work = kmalloc(sizeof(*work), GFP_ATOMIC);
-	if (work) {
-		bdi_work_init(work, args);
-		bdi_queue_work(bdi, work);
-	} else {
-		struct bdi_writeback *wb = &bdi->wb;
-
-		if (wb->task)
-			wake_up_process(wb->task);
+	work = kzalloc(sizeof(*work), GFP_ATOMIC);
+	if (!work) {
+		if (bdi->wb.task)
+			wake_up_process(bdi->wb.task);
+		return;
 	}
-}
-
-/**
- * bdi_queue_work_onstack - start and wait for writeback
- * @sb: write inodes from this super_block
- *
- * Description:
- *   This function initiates writeback and waits for the operation to
- *   complete. Callers must hold the sb s_umount semaphore for
- *   reading, to avoid having the super disappear before we are done.
- */
-static void bdi_queue_work_onstack(struct wb_writeback_args *args)
-{
-	struct bdi_work work;
 
-	bdi_work_init(&work, args);
-	__set_bit(WS_ONSTACK, &work.state);
+	work->sync_mode	= WB_SYNC_NONE;
+	work->nr_pages	= nr_pages;
+	work->range_cyclic = range_cyclic;
+	work->for_background = for_background;
 
-	bdi_queue_work(args->sb->s_bdi, &work);
-	bdi_wait_on_work_done(&work);
+	bdi_queue_work(bdi, work);
 }
 
 /**

@@ -210,13 +121,7 @@ static void bdi_queue_work_onstack(struc
  */
 void bdi_start_writeback(struct backing_dev_info *bdi, long nr_pages)
 {
-	struct wb_writeback_args args = {
-		.sync_mode	= WB_SYNC_NONE,
-		.nr_pages	= nr_pages,
-		.range_cyclic	= 1,
-	};
-
-	bdi_alloc_queue_work(bdi, &args);
+	__bdi_start_writeback(bdi, nr_pages, true, false);
 }
 
 /**

@@ -230,13 +135,7 @@ void bdi_start_writeback(struct backing_
  */
 void bdi_start_background_writeback(struct backing_dev_info *bdi)
 {
-	struct wb_writeback_args args = {
-		.sync_mode	= WB_SYNC_NONE,
-		.nr_pages	= LONG_MAX,
-		.for_background = 1,
-		.range_cyclic	= 1,
-	};
-	bdi_alloc_queue_work(bdi, &args);
+	__bdi_start_writeback(bdi, LONG_MAX, true, true);
 }
 
 /*

@@ -694,14 +593,14 @@ static inline bool over_bground_thresh(v
  * all dirty pages if they are all attached to "old" mappings.
  */
 static long wb_writeback(struct bdi_writeback *wb,
-			 struct wb_writeback_args *args)
+			 struct wb_writeback_work *work)
 {
 	struct writeback_control wbc = {
-		.sync_mode		= args->sync_mode,
+		.sync_mode		= work->sync_mode,
 		.older_than_this	= NULL,
-		.for_kupdate		= args->for_kupdate,
-		.for_background		= args->for_background,
-		.range_cyclic		= args->range_cyclic,
+		.for_kupdate		= work->for_kupdate,
+		.for_background		= work->for_background,
+		.range_cyclic		= work->range_cyclic,
 	};
 	unsigned long oldest_jif;
 	long wrote = 0;

@@ -721,24 +620,24 @@ static long wb_writeback(struct bdi_writ
 		/*
 		 * Stop writeback when nr_pages has been consumed
 		 */
-		if (args->nr_pages <= 0)
+		if (work->nr_pages <= 0)
 			break;
 
 		/*
 		 * For background writeout, stop when we are below the
 		 * background dirty threshold
 		 */
-		if (args->for_background && !over_bground_thresh())
+		if (work->for_background && !over_bground_thresh())
 			break;
 
 		wbc.more_io = 0;
 		wbc.nr_to_write = MAX_WRITEBACK_PAGES;
 		wbc.pages_skipped = 0;
-		if (args->sb)
-			__writeback_inodes_sb(args->sb, wb, &wbc);
+		if (work->sb)
+			__writeback_inodes_sb(work->sb, wb, &wbc);
 		else
 			writeback_inodes_wb(wb, &wbc);
-		args->nr_pages -= MAX_WRITEBACK_PAGES - wbc.nr_to_write;
+		work->nr_pages -= MAX_WRITEBACK_PAGES - wbc.nr_to_write;
 		wrote += MAX_WRITEBACK_PAGES - wbc.nr_to_write;
 
 		/*

@@ -774,31 +673,21 @@ static long wb_writeback(struct bdi_writ
 }
 
 /*
- * Return the next bdi_work struct that hasn't been processed by this
- * wb thread yet. ->seen is initially set for each thread that exists
- * for this device, when a thread first notices a piece of work it
- * clears its bit. Depending on writeback type, the thread will notify
- * completion on either receiving the work (WB_SYNC_NONE) or after
- * it is done (WB_SYNC_ALL).
+ * Return the next wb_writeback_work struct that hasn't been processed yet.
  */
-static struct bdi_work *get_next_work_item(struct backing_dev_info *bdi,
-					   struct bdi_writeback *wb)
+static struct wb_writeback_work *
+get_next_work_item(struct backing_dev_info *bdi, struct bdi_writeback *wb)
 {
-	struct bdi_work *work, *ret = NULL;
-
-	rcu_read_lock();
+	struct wb_writeback_work *work = NULL;
 
-	list_for_each_entry_rcu(work, &bdi->work_list, list) {
-		if (!test_bit(wb->nr, &work->seen))
-			continue;
-		clear_bit(wb->nr, &work->seen);
-
-		ret = work;
-		break;
+	spin_lock(&bdi->wb_lock);
+	if (!list_empty(&bdi->work_list)) {
+		work = list_entry(bdi->work_list.next,
+				  struct wb_writeback_work, list);
+		list_del_init(&work->list);
 	}
-
-	rcu_read_unlock();
-	return ret;
+	spin_unlock(&bdi->wb_lock);
+	return work;
 }
 
 static long wb_check_old_data_flush(struct bdi_writeback *wb)

@@ -823,14 +712,14 @@ static long wb_check_old_data_flush(stru
 			(inodes_stat.nr_inodes - inodes_stat.nr_unused);
 
 	if (nr_pages) {
-		struct wb_writeback_args args = {
+		struct wb_writeback_work work = {
 			.nr_pages	= nr_pages,
 			.sync_mode	= WB_SYNC_NONE,
 			.for_kupdate	= 1,
 			.range_cyclic	= 1,
 		};
 
-		return wb_writeback(wb, &args);
+		return wb_writeback(wb, &work);
 	}
 
 	return 0;

@@ -842,33 +731,27 @@ static long wb_check_old_data_flush(stru
 long wb_do_writeback(struct bdi_writeback *wb, int force_wait)
 {
 	struct backing_dev_info *bdi = wb->bdi;
-	struct bdi_work *work;
+	struct wb_writeback_work *work;
 	long wrote = 0;
 
 	while ((work = get_next_work_item(bdi, wb)) != NULL) {
-		struct wb_writeback_args args = work->args;
-
 		/*
 		 * Override sync mode, in case we must wait for completion
+		 * because this thread is exiting now.
 		 */
 		if (force_wait)
-			work->args.sync_mode = args.sync_mode = WB_SYNC_ALL;
+			work->sync_mode = WB_SYNC_ALL;
 
-		/*
-		 * If this isn't a data integrity operation, just notify
-		 * that we have seen this work and we are now starting it.
-		 */
-		if (!test_bit(WS_ONSTACK, &work->state))
-			wb_clear_pending(wb, work);
-
-		wrote += wb_writeback(wb, &args);
+		wrote += wb_writeback(wb, work);
 
 		/*
-		 * This is a data integrity writeback, so only do the
-		 * notification when we have completed the work.
+		 * Notify the caller of completion if this is a synchronous
+		 * work item, otherwise just free it.
 		 */
-		if (test_bit(WS_ONSTACK, &work->state))
-			wb_clear_pending(wb, work);
+		if (work->done)
+			complete(work->done);
+		else
+			kfree(work);
 	}
 
 	/*

@@ -931,14 +814,9 @@ int bdi_writeback_task(struct bdi_writeb
 void wakeup_flusher_threads(long nr_pages)
 {
 	struct backing_dev_info *bdi;
-	struct wb_writeback_args args = {
-		.sync_mode	= WB_SYNC_NONE,
-	};
 
-	if (nr_pages) {
-		args.nr_pages = nr_pages;
-	} else {
-		args.nr_pages = global_page_state(NR_FILE_DIRTY) +
+	if (!nr_pages) {
+		nr_pages = global_page_state(NR_FILE_DIRTY) +
 				global_page_state(NR_UNSTABLE_NFS);
 	}
 

@@ -946,7 +824,7 @@ void wakeup_flusher_threads(long nr_page
 	list_for_each_entry_rcu(bdi, &bdi_list, bdi_list) {
 		if (!bdi_has_dirty_io(bdi))
 			continue;
-		bdi_alloc_queue_work(bdi, &args);
+		__bdi_start_writeback(bdi, nr_pages, false, false);
 	}
 	rcu_read_unlock();
 }

@@ -1155,17 +1033,20 @@ void writeback_inodes_sb(struct super_bl
 {
 	unsigned long nr_dirty = global_page_state(NR_FILE_DIRTY);
 	unsigned long nr_unstable = global_page_state(NR_UNSTABLE_NFS);
-	struct wb_writeback_args args = {
+	DECLARE_COMPLETION_ONSTACK(done);
+	struct wb_writeback_work work = {
 		.sb		= sb,
 		.sync_mode	= WB_SYNC_NONE,
+		.done		= &done,
 	};
 
 	WARN_ON(!rwsem_is_locked(&sb->s_umount));
 
-	args.nr_pages = nr_dirty + nr_unstable +
+	work.nr_pages = nr_dirty + nr_unstable +
 			(inodes_stat.nr_inodes - inodes_stat.nr_unused);
 
-	bdi_queue_work_onstack(&args);
+	bdi_queue_work(sb->s_bdi, &work);
+	wait_for_completion(&done);
 }
 EXPORT_SYMBOL(writeback_inodes_sb);
 

@@ -1197,16 +1078,20 @@ EXPORT_SYMBOL(writeback_inodes_sb_if_idl
  */
 void sync_inodes_sb(struct super_block *sb)
 {
-	struct wb_writeback_args args = {
+	DECLARE_COMPLETION_ONSTACK(done);
+	struct wb_writeback_work work = {
 		.sb		= sb,
 		.sync_mode	= WB_SYNC_ALL,
 		.nr_pages	= LONG_MAX,
 		.range_cyclic	= 0,
+		.done		= &done,
 	};
 
 	WARN_ON(!rwsem_is_locked(&sb->s_umount));
 
-	bdi_queue_work_onstack(&args);
+	bdi_queue_work(sb->s_bdi, &work);
+	wait_for_completion(&done);
+
 	wait_sb_inodes(sb);
 }
 EXPORT_SYMBOL(sync_inodes_sb);

Index: linux-2.6/include/linux/backing-dev.h
===================================================================
@@ -82,8 +82,6 @@ struct backing_dev_info {
 	struct bdi_writeback wb;  /* default writeback info for this bdi */
 	spinlock_t wb_lock;	  /* protects update side of wb_list */
 	struct list_head wb_list; /* the flusher threads hanging off this bdi */
-	unsigned long wb_mask;	  /* bitmask of registered tasks */
-	unsigned int wb_cnt;	  /* number of registered tasks */
 
 	struct list_head work_list;
 

Index: linux-2.6/mm/backing-dev.c
===================================================================
@@ -104,15 +104,13 @@ static int bdi_debug_stats_show(struct s
 		   "b_more_io:        %8lu\n"
 		   "bdi_list:         %8u\n"
 		   "state:            %8lx\n"
-		   "wb_mask:          %8lx\n"
-		   "wb_list:          %8u\n"
-		   "wb_cnt:           %8u\n",
+		   "wb_list:          %8u\n",
 		   (unsigned long) K(bdi_stat(bdi, BDI_WRITEBACK)),
 		   (unsigned long) K(bdi_stat(bdi, BDI_RECLAIMABLE)),
 		   K(bdi_thresh), K(dirty_thresh),
 		   K(background_thresh), nr_wb, nr_dirty, nr_io, nr_more_io,
-		   !list_empty(&bdi->bdi_list), bdi->state, bdi->wb_mask,
-		   !list_empty(&bdi->wb_list), bdi->wb_cnt);
+		   !list_empty(&bdi->bdi_list), bdi->state,
+		   !list_empty(&bdi->wb_list));
 #undef K
 
 	return 0;

@@ -674,12 +672,6 @@ int bdi_init(struct backing_dev_info *bd
 
 	bdi_wb_init(&bdi->wb, bdi);
 
-	/*
-	 * Just one thread support for now, hard code mask and count
-	 */
-	bdi->wb_mask = 1;
-	bdi->wb_cnt = 1;
-
 	for (i = 0; i < NR_BDI_STAT_ITEMS; i++) {
 		err = percpu_counter_init(&bdi->bdi_stat[i], 0);
 		if (err)