lockd: convert nsm_mutex to a spinlock

There's no reason for a mutex here, except to allow an allocation under
the lock, which we can avoid with the usual trick of preallocating
memory for the new object and freeing it if it turns out to be
unnecessary.
Signed-off-by: NJ. Bruce Fields <bfields@citi.umich.edu>

fs/lockd/host.c

@@ -457,7 +457,7 @@ nlm_gc_hosts(void)
  * Manage NSM handles
  */
 static LIST_HEAD(nsm_handles);
-static DEFINE_MUTEX(nsm_mutex);
+static DEFINE_SPINLOCK(nsm_lock);
 
 static struct nsm_handle *
 __nsm_find(const struct sockaddr_in *sin,
@@ -479,7 +479,8 @@ __nsm_find(const struct sockaddr_in *sin,
 		return NULL;
 	}
 
-	mutex_lock(&nsm_mutex);
+retry:
+	spin_lock(&nsm_lock);
 	list_for_each_entry(pos, &nsm_handles, sm_link) {
 
 		if (hostname && nsm_use_hostnames) {
@@ -489,28 +490,32 @@ __nsm_find(const struct sockaddr_in *sin,
 		} else if (!nlm_cmp_addr(&pos->sm_addr, sin))
 			continue;
 		atomic_inc(&pos->sm_count);
+		kfree(nsm);
 		nsm = pos;
-		goto out;
+		goto found;
 	}
-
-	if (!create) {
-		nsm = NULL;
-		goto out;
+	if (nsm) {
+		list_add(&nsm->sm_link, &nsm_handles);
+		goto found;
 	}
+	spin_unlock(&nsm_lock);
+
+	if (!create)
+		return NULL;
 
 	nsm = kzalloc(sizeof(*nsm) + hostname_len + 1, GFP_KERNEL);
 	if (nsm == NULL)
-		goto out;
+		return NULL;
+
 	nsm->sm_addr = *sin;
 	nsm->sm_name = (char *) (nsm + 1);
 	memcpy(nsm->sm_name, hostname, hostname_len);
 	nsm->sm_name[hostname_len] = '\0';
 	atomic_set(&nsm->sm_count, 1);
+	goto retry;
 
-	list_add(&nsm->sm_link, &nsm_handles);
-
-out:
-	mutex_unlock(&nsm_mutex);
+found:
+	spin_unlock(&nsm_lock);
 	return nsm;
 }
 
@@ -529,10 +534,9 @@ nsm_release(struct nsm_handle *nsm)
 {
 	if (!nsm)
 		return;
-	mutex_lock(&nsm_mutex);
-	if (atomic_dec_and_test(&nsm->sm_count)) {
+	if (atomic_dec_and_lock(&nsm->sm_count, &nsm_lock)) {
 		list_del(&nsm->sm_link);
+		spin_unlock(&nsm_lock);
 		kfree(nsm);
 	}
-	mutex_unlock(&nsm_mutex);
 }