rbd: switch to ida for rbd id assignments

Currently rbd ids are allocated using an atomic variable that keeps
track of the highest id currently in use and each new id is simply one
more than the value of that variable.  That's nice and cheap, but it
does mean that rbd ids are allowed to grow boundlessly, and, more
importantly, it's completely unpredictable.  So, in preparation for
single-major device number allocation scheme, which is going to
establish and rely on a constant mapping between rbd ids and device
numbers, switch to ida for rbd id assignments.
Signed-off-by: Ilya Dryomov <ilya.dryomov@inktank.com>
Reviewed-by: Alex Elder <elder@linaro.org>
Reviewed-by: Josh Durgin <josh.durgin@inktank.com>

drivers/block/rbd.c

@@ -41,6 +41,7 @@
 #include <linux/fs.h>
 #include <linux/blkdev.h>
 #include <linux/slab.h>
+#include <linux/idr.h>
 
 #include "rbd_types.h"
 
@@ -385,6 +386,8 @@ static struct kmem_cache	*rbd_img_request_cache;
 static struct kmem_cache	*rbd_obj_request_cache;
 static struct kmem_cache	*rbd_segment_name_cache;
 
+static DEFINE_IDA(rbd_dev_id_ida);
+
 static int rbd_img_request_submit(struct rbd_img_request *img_request);
 
 static void rbd_dev_device_release(struct device *dev);
@@ -4371,20 +4374,27 @@ static void rbd_bus_del_dev(struct rbd_device *rbd_dev)
 	device_unregister(&rbd_dev->dev);
 }
 
-static atomic64_t rbd_dev_id_max = ATOMIC64_INIT(0);
-
 /*
  * Get a unique rbd identifier for the given new rbd_dev, and add
- * the rbd_dev to the global list.  The minimum rbd id is 1.
+ * the rbd_dev to the global list.
  */
-static void rbd_dev_id_get(struct rbd_device *rbd_dev)
+static int rbd_dev_id_get(struct rbd_device *rbd_dev)
 {
-	rbd_dev->dev_id = atomic64_inc_return(&rbd_dev_id_max);
+	int new_dev_id;
+
+	new_dev_id = ida_simple_get(&rbd_dev_id_ida, 0, 0, GFP_KERNEL);
+	if (new_dev_id < 0)
+		return new_dev_id;
+
+	rbd_dev->dev_id = new_dev_id;
 
 	spin_lock(&rbd_dev_list_lock);
 	list_add_tail(&rbd_dev->node, &rbd_dev_list);
 	spin_unlock(&rbd_dev_list_lock);
+
 	dout("rbd_dev %p given dev id %d\n", rbd_dev, rbd_dev->dev_id);
+
+	return 0;
 }
 
 /*
@@ -4393,48 +4403,13 @@ static void rbd_dev_id_get(struct rbd_device *rbd_dev)
  */
 static void rbd_dev_id_put(struct rbd_device *rbd_dev)
 {
-	struct list_head *tmp;
-	int rbd_id = rbd_dev->dev_id;
-	int max_id;
-
-	rbd_assert(rbd_id > 0);
-
-	dout("rbd_dev %p released dev id %d\n", rbd_dev, rbd_dev->dev_id);
 	spin_lock(&rbd_dev_list_lock);
 	list_del_init(&rbd_dev->node);
-
-	/*
-	 * If the id being "put" is not the current maximum, there
-	 * is nothing special we need to do.
-	 */
-	if (rbd_id != atomic64_read(&rbd_dev_id_max)) {
-		spin_unlock(&rbd_dev_list_lock);
-		return;
-	}
-
-	/*
-	 * We need to update the current maximum id.  Search the
-	 * list to find out what it is.  We're more likely to find
-	 * the maximum at the end, so search the list backward.
-	 */
-	max_id = 0;
-	list_for_each_prev(tmp, &rbd_dev_list) {
-		struct rbd_device *rbd_dev;
-
-		rbd_dev = list_entry(tmp, struct rbd_device, node);
-		if (rbd_dev->dev_id > max_id)
-			max_id = rbd_dev->dev_id;
-	}
 	spin_unlock(&rbd_dev_list_lock);
 
-	/*
-	 * The max id could have been updated by rbd_dev_id_get(), in
-	 * which case it now accurately reflects the new maximum.
-	 * Be careful not to overwrite the maximum value in that
-	 * case.
-	 */
-	atomic64_cmpxchg(&rbd_dev_id_max, rbd_id, max_id);
-	dout("  max dev id has been reset\n");
+	ida_simple_remove(&rbd_dev_id_ida, rbd_dev->dev_id);
+
+	dout("rbd_dev %p released dev id %d\n", rbd_dev, rbd_dev->dev_id);
 }
 
 /*
@@ -4857,10 +4832,12 @@ static int rbd_dev_device_setup(struct rbd_device *rbd_dev)
 {
 	int ret;
 
-	/* generate unique id: find highest unique id, add one */
-	rbd_dev_id_get(rbd_dev);
+	/* Get an id and fill in device name. */
+
+	ret = rbd_dev_id_get(rbd_dev);
+	if (ret)
+		return ret;
 
-	/* Fill in the device name, now that we have its id. */
 	BUILD_BUG_ON(DEV_NAME_LEN
 			< sizeof (RBD_DRV_NAME) + MAX_INT_FORMAT_WIDTH);
 	sprintf(rbd_dev->name, "%s%d", RBD_DRV_NAME, rbd_dev->dev_id);