nbd: only return usable devices from nbd_find_unused

Device marked as NBD_DESTROY_ON_DISCONNECT can and should be skipped
given that they won't survive the disconnect.  So skip them and try
to grab a reference directly and just continue if the the devices
is being torn down or created and thus has a zero refcount.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20210825163108.50713-6-hch@lst.de
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

diff --git a/drivers/block/nbd.c b/drivers/block/nbd.c
index 6a832bf..70dc9d8 100644 (file)
--- a/drivers/block/nbd.c
+++ b/drivers/block/nbd.c
@@ -1790,16 +1790,20 @@ out:
        return ERR_PTR(err);
 }
 
-static struct nbd_device *nbd_find_unused(void)
+static struct nbd_device *nbd_find_get_unused(void)
 {
        struct nbd_device *nbd;
        int id;
 
        lockdep_assert_held(&nbd_index_mutex);
 
-       idr_for_each_entry(&nbd_index_idr, nbd, id)
-               if (!refcount_read(&nbd->config_refs))
+       idr_for_each_entry(&nbd_index_idr, nbd, id) {
+               if (refcount_read(&nbd->config_refs) ||
+                   test_bit(NBD_DESTROY_ON_DISCONNECT, &nbd->flags))
+                       continue;
+               if (refcount_inc_not_zero(&nbd->refs))
                        return nbd;
+       }
 
        return NULL;
 }
@@ -1873,10 +1877,10 @@ static int nbd_genl_connect(struct sk_buff *skb, struct genl_info *info)
 again:
        mutex_lock(&nbd_index_mutex);
        if (index == -1)
-               nbd = nbd_find_unused();
+               nbd = nbd_find_get_unused();
        else
                nbd = idr_find(&nbd_index_idr, index);
-       if (nbd) {
+       if (nbd && index != -1) {
                if (test_bit(NBD_DESTROY_ON_DISCONNECT, &nbd->flags) &&
                    test_bit(NBD_DISCONNECT_REQUESTED, &nbd->flags)) {
                        nbd->destroy_complete = &destroy_complete;
@@ -1889,8 +1893,6 @@ again:
 
                if (!refcount_inc_not_zero(&nbd->refs)) {
                        mutex_unlock(&nbd_index_mutex);
-                       if (index == -1)
-                               goto again;
                        pr_err("nbd: device at index %d is going down\n",
                                index);
                        return -EINVAL;