[PATCH] Enhance new plugging support to support general callbacks.

[NeilBrown]

md/raid requires an unplug callback, but as it does not uses
requests the current code cannot provide one.

So allow arbitrary callbacks to be attached to the blk_plug.

Cc: Jens Axboe <jaxboe@xxxxxxxxxxxx>
Signed-off-by: NeilBrown <neilb@xxxxxxx>
---
 block/blk-core.c       |   13 +++++++++++++
 include/linux/blkdev.h |    7 ++++++-
 2 files changed, 19 insertions(+), 1 deletions(-)

diff --git a/block/blk-core.c b/block/blk-core.c
index 725091d..273d60b 100644
--- a/block/blk-core.c
+++ b/block/blk-core.c
@@ -2644,6 +2644,7 @@ void blk_start_plug(struct blk_plug *plug)
 
 	plug->magic = PLUG_MAGIC;
 	INIT_LIST_HEAD(&plug->list);
+	INIT_LIST_HEAD(&plug->cb_list);
 	plug->should_sort = 0;
 
 	/*
@@ -2717,9 +2718,21 @@ static void flush_plug_list(struct blk_plug *plug)
 	local_irq_restore(flags);
 }
 
+static void flush_plug_callbacks(struct blk_plug *plug)
+{
+	while (!list_empty(&plug->cb_list)) {
+		struct blk_plug_cb *cb = list_first_entry(&plug->cb_list,
+							  struct blk_plug_cb,
+							  list);
+		list_del(&cb->list);
+		cb->callback(cb);
+	}
+}
+
 static void __blk_finish_plug(struct task_struct *tsk, struct blk_plug *plug)
 {
 	flush_plug_list(plug);
+	flush_plug_callbacks(plug);
 
 	if (plug == tsk->plug)
 		tsk->plug = NULL;
diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
index 32176cc..3e5e604 100644
--- a/include/linux/blkdev.h
+++ b/include/linux/blkdev.h
@@ -857,8 +857,13 @@ extern void blk_put_queue(struct request_queue *);
 struct blk_plug {
 	unsigned long magic;
 	struct list_head list;
+	struct list_head cb_list;
 	unsigned int should_sort;
 };
+struct blk_plug_cb {
+	struct list_head list;
+	void (*callback)(struct blk_plug_cb *);
+};
 
 extern void blk_start_plug(struct blk_plug *);
 extern void blk_finish_plug(struct blk_plug *);
@@ -876,7 +881,7 @@ static inline bool blk_needs_flush_plug(struct task_struct *tsk)
 {
 	struct blk_plug *plug = tsk->plug;
 
-	return plug && !list_empty(&plug->list);
+	return plug && (!list_empty(&plug->list) || !list_empty(&plug->cb_list));
 }
 
 /*
-- 
1.7.3.4

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