rbd: restore previous rbd id sequence behavior

It used to be that selecting a new unique identifier for an added
rbd device required searching all existing ones to find the highest
id is used.  A recent change made that unnecessary, but made it
so that id's used were monotonically non-decreasing.  It's a bit
more pleasant to have smaller rbd id's though, and this change
makes ids get allocated as they were before--each new id is one more
than the maximum currently in use.

Signed-off-by: Alex Elder <elder@dreamhost.com>
Signed-off-by: Sage Weil <sage@newdream.net>

1 files changed, 34 insertions, 6 deletions

diff --git a/drivers/block/rbd.c b/drivers/block/rbd.c
index e7727e8337fc..9ac1484a95ad 100644
--- a/drivers/block/rbd.c
+++ b/drivers/block/rbd.c
@@ -2172,18 +2172,46 @@ static void rbd_id_get(struct rbd_device *rbd_dev)
  */
 static void rbd_id_put(struct rbd_device *rbd_dev)
 {
-        BUG_ON(rbd_dev->id < 1);
+        struct list_head *tmp;
+        int rbd_id = rbd_dev->id;
+        int max_id;
+
+        BUG_ON(rbd_id < 1);
 
         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_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_id > max_id)
+                        max_id = rbd_id;
+        }
         spin_unlock(&rbd_dev_list_lock);
 
         /*
-         * New id's are always one more than the current maximum.
-         * If the id being "put" *is* that maximum, decrement the
-         * maximum so the next one requested just reuses this one.
+         * The max id could have been updated by rbd_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_id_max, rbd_dev->id, rbd_dev->id - 1);
+        atomic64_cmpxchg(&rbd_id_max, rbd_id, max_id);
 }
 
 static ssize_t rbd_add(struct bus_type *bus,
@@ -2220,7 +2248,7 @@ static ssize_t rbd_add(struct bus_type *bus,
 
         init_rwsem(&rbd_dev->header.snap_rwsem);
 
-        /* generate unique id: one more than highest used so far */
+        /* generate unique id: find highest unique id, add one */
         rbd_id_get(rbd_dev);
 
         /* parse add command */