On Tue, Feb 28, 2012 at 7:40 PM, Alex Elder <elder@xxxxxxxxxxxxx> wrote:
> 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.
Oh, so ignore my previous comment, though in any case we can still
exhaust the 31 bits:
loop through:
add A
add B
remove A
add C
remove B
add A
remove C
add B
etc..
>
> Signed-off-by: Alex Elder <elder@xxxxxxxxxxxxx>
> ---
> drivers/block/rbd.c | 40 ++++++++++++++++++++++++++++++++++------
> 1 files changed, 34 insertions(+), 6 deletions(-)
>
> diff --git a/drivers/block/rbd.c b/drivers/block/rbd.c
> index 042377b..4c5bb39 100644
> --- a/drivers/block/rbd.c
> +++ b/drivers/block/rbd.c
> @@ -2171,18 +2171,46 @@ 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 != atomic_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);
So we still need to either use 64 bit counter, or think of a better
scheme to allocate ids. Maybe using some sorted data structure here?
>
> /*
> - * 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.
> */
> - atomic_cmpxchg(&rbd_id_max, rbd_dev->id, rbd_dev->id - 1);
> + atomic_cmpxchg(&rbd_id_max, rbd_id, max_id);
> }
>
> static ssize_t rbd_add(struct bus_type *bus,
> @@ -2217,7 +2245,7 @@ static ssize_t rbd_add(struct bus_type *bus,
> INIT_LIST_HEAD(&rbd_dev->node);
> INIT_LIST_HEAD(&rbd_dev->snaps);
>
> - /* 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 */
> --
> 1.7.5.4
>
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