Add a workqueue so that I/O can be queued up to be flushed from a
separate thread when necessary.

This is necessary because we will want to flush I/O from end_io()
which can be called in irq context, and flushing I/O can cause the
caller to sleep.

A new per-snapshot spinlock pe_lock is introduced to protect
queued_bios.

Index: linux-2.6.16.i686/drivers/md/dm-snap.c
===================================================================
--- linux-2.6.16.i686.orig/drivers/md/dm-snap.c	2006-04-24 13:28:25.000000000 +0100
+++ linux-2.6.16.i686/drivers/md/dm-snap.c	2006-04-25 08:58:17.000000000 +0100
@@ -38,6 +38,9 @@
  */
 #define SNAPSHOT_PAGES 256
 
+struct workqueue_struct *ksnapd;
+static void flush_queued_bios(void *data);
+
 struct pending_exception {
 	struct exception e;
 
@@ -490,6 +493,7 @@ static int snapshot_ctr(struct dm_target
 	s->active = 0;
 	s->last_percent = 0;
 	init_rwsem(&s->lock);
+	spin_lock_init(&s->pe_lock);
 	s->table = ti->table;
 
 	/* Allocate hash table for COW data */
@@ -525,6 +529,9 @@ static int snapshot_ctr(struct dm_target
 		goto bad6;
 	}
 
+	bio_list_init(&s->queued_bios);
+	INIT_WORK(&s->queued_bios_work, flush_queued_bios, s);
+
 	/* Add snapshot to the list of snapshots for this origin */
 	/* Exceptions aren't triggered till snapshot_resume() is called */
 	if (register_snapshot(s)) {
@@ -563,6 +570,8 @@ static void snapshot_dtr(struct dm_targe
 {
 	struct dm_snapshot *s = (struct dm_snapshot *) ti->private;
 
+	flush_workqueue(ksnapd);
+
 	/* Prevent further origin writes from using this snapshot. */
 	/* After this returns there can be no new kcopyd jobs. */
 	unregister_snapshot(s);
@@ -596,6 +605,19 @@ static void flush_bios(struct bio *bio)
 	}
 }
 
+static void flush_queued_bios(void *data)
+{
+	struct dm_snapshot *s = (struct dm_snapshot *) data;
+	struct bio *queued_bios;
+	unsigned long flags;
+
+	spin_lock_irqsave(&s->pe_lock, flags);
+	queued_bios = bio_list_get(&s->queued_bios);
+	spin_unlock_irqrestore(&s->pe_lock, flags);
+
+	flush_bios(queued_bios);
+}
+
 /*
  * Error a list of buffers.
  */
@@ -1241,8 +1263,17 @@ static int __init dm_snapshot_init(void)
 		goto bad5;
 	}
 
+	ksnapd = create_singlethread_workqueue("ksnapd");
+	if (!ksnapd) {
+		DMERR("Failed to create ksnapd workqueue.");
+		r = -ENOMEM;
+		goto bad6;
+	}
+
 	return 0;
 
+      bad6:
+	mempool_destroy(pending_pool);
       bad5:
 	kmem_cache_destroy(pending_cache);
       bad4:
@@ -1260,6 +1291,8 @@ static void __exit dm_snapshot_exit(void
 {
 	int r;
 
+	destroy_workqueue(ksnapd);
+
 	r = dm_unregister_target(&snapshot_target);
 	if (r)
 		DMERR("snapshot unregister failed %d", r);
Index: linux-2.6.16.i686/drivers/md/dm-snap.h
===================================================================
--- linux-2.6.16.i686.orig/drivers/md/dm-snap.h	2006-04-24 13:28:21.000000000 +0100
+++ linux-2.6.16.i686/drivers/md/dm-snap.h	2006-04-24 13:28:26.000000000 +0100
@@ -10,7 +10,9 @@
 #define DM_SNAPSHOT_H
 
 #include "dm.h"
+#include "dm-bio-list.h"
 #include <linux/blkdev.h>
+#include <linux/workqueue.h>
 
 struct exception_table {
 	uint32_t hash_mask;
@@ -112,10 +114,20 @@ struct dm_snapshot {
 	struct exception_table pending;
 	struct exception_table complete;
 
+	/*
+	 * pe_lock protects all pending_exception operations and access
+	 * as well as the snapshot_bios list.
+	 */
+	spinlock_t pe_lock;
+
 	/* The on disk metadata handler */
 	struct exception_store store;
 
 	struct kcopyd_client *kcopyd_client;
+
+	/* Queue of snapshot writes for ksnapd to flush */
+	struct bio_list queued_bios;
+	struct work_struct queued_bios_work;
 };
 
 /*