SUNRPC: Remove rpc_authflavor_lock in favour of RCU locking

Module removal is RCU safe by design, so we really have no need to
lock the auth_flavors[] array. Substitute a lockless scheme to
add/remove entries in the array, and then use rcu.

Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>

1 files changed, 62 insertions, 66 deletions

diff --git a/net/sunrpc/auth.c b/net/sunrpc/auth.c
index 59df5cdba0ac..32985aa515be 100644
--- a/net/sunrpc/auth.c
+++ b/net/sunrpc/auth.c
@@ -30,10 +30,9 @@ struct rpc_cred_cache {
 
 static unsigned int auth_hashbits = RPC_CREDCACHE_DEFAULT_HASHBITS;
 
-static DEFINE_SPINLOCK(rpc_authflavor_lock);
-static const struct rpc_authops *auth_flavors[RPC_AUTH_MAXFLAVOR] = {
-        &authnull_ops,          /* AUTH_NULL */
-        &authunix_ops,          /* AUTH_UNIX */
+static const struct rpc_authops __rcu *auth_flavors[RPC_AUTH_MAXFLAVOR] = {
+        [RPC_AUTH_NULL] = (const struct rpc_authops __force __rcu *)&authnull_ops,
+        [RPC_AUTH_UNIX] = (const struct rpc_authops __force __rcu *)&authunix_ops,
         NULL,                   /* others can be loadable modules */
 };
 
@@ -93,39 +92,65 @@ pseudoflavor_to_flavor(u32 flavor) {
 int
 rpcauth_register(const struct rpc_authops *ops)
 {
+        const struct rpc_authops *old;
         rpc_authflavor_t flavor;
-        int ret = -EPERM;
 
         if ((flavor = ops->au_flavor) >= RPC_AUTH_MAXFLAVOR)
                 return -EINVAL;
-        spin_lock(&rpc_authflavor_lock);
-        if (auth_flavors[flavor] == NULL) {
-                auth_flavors[flavor] = ops;
-                ret = 0;
-        }
-        spin_unlock(&rpc_authflavor_lock);
-        return ret;
+        old = cmpxchg((const struct rpc_authops ** __force)&auth_flavors[flavor], NULL, ops);
+        if (old == NULL || old == ops)
+                return 0;
+        return -EPERM;
 }
 EXPORT_SYMBOL_GPL(rpcauth_register);
 
 int
 rpcauth_unregister(const struct rpc_authops *ops)
 {
+        const struct rpc_authops *old;
         rpc_authflavor_t flavor;
-        int ret = -EPERM;
 
         if ((flavor = ops->au_flavor) >= RPC_AUTH_MAXFLAVOR)
                 return -EINVAL;
-        spin_lock(&rpc_authflavor_lock);
-        if (auth_flavors[flavor] == ops) {
-                auth_flavors[flavor] = NULL;
-                ret = 0;
-        }
-        spin_unlock(&rpc_authflavor_lock);
-        return ret;
+
+        old = cmpxchg((const struct rpc_authops ** __force)&auth_flavors[flavor], ops, NULL);
+        if (old == ops || old == NULL)
+                return 0;
+        return -EPERM;
 }
 EXPORT_SYMBOL_GPL(rpcauth_unregister);
 
+static const struct rpc_authops *
+rpcauth_get_authops(rpc_authflavor_t flavor)
+{
+        const struct rpc_authops *ops;
+
+        if (flavor >= RPC_AUTH_MAXFLAVOR)
+                return NULL;
+
+        rcu_read_lock();
+        ops = rcu_dereference(auth_flavors[flavor]);
+        if (ops == NULL) {
+                rcu_read_unlock();
+                request_module("rpc-auth-%u", flavor);
+                rcu_read_lock();
+                ops = rcu_dereference(auth_flavors[flavor]);
+                if (ops == NULL)
+                        goto out;
+        }
+        if (!try_module_get(ops->owner))
+                ops = NULL;
+out:
+        rcu_read_unlock();
+        return ops;
+}
+
+static void
+rpcauth_put_authops(const struct rpc_authops *ops)
+{
+        module_put(ops->owner);
+}
+
 /**
  * rpcauth_get_pseudoflavor - check if security flavor is supported
  * @flavor: a security flavor
@@ -138,25 +163,16 @@ EXPORT_SYMBOL_GPL(rpcauth_unregister);
 rpc_authflavor_t
 rpcauth_get_pseudoflavor(rpc_authflavor_t flavor, struct rpcsec_gss_info *info)
 {
-        const struct rpc_authops *ops;
+        const struct rpc_authops *ops = rpcauth_get_authops(flavor);
         rpc_authflavor_t pseudoflavor;
 
-        ops = auth_flavors[flavor];
-        if (ops == NULL)
-                request_module("rpc-auth-%u", flavor);
-        spin_lock(&rpc_authflavor_lock);
-        ops = auth_flavors[flavor];
-        if (ops == NULL || !try_module_get(ops->owner)) {
-                spin_unlock(&rpc_authflavor_lock);
+        if (!ops)
                 return RPC_AUTH_MAXFLAVOR;
-        }
-        spin_unlock(&rpc_authflavor_lock);
-
         pseudoflavor = flavor;
         if (ops->info2flavor != NULL)
                 pseudoflavor = ops->info2flavor(info);
 
-        module_put(ops->owner);
+        rpcauth_put_authops(ops);
         return pseudoflavor;
 }
 EXPORT_SYMBOL_GPL(rpcauth_get_pseudoflavor);
@@ -176,25 +192,15 @@ rpcauth_get_gssinfo(rpc_authflavor_t pseudoflavor, struct rpcsec_gss_info *info)
         const struct rpc_authops *ops;
         int result;
 
-        if (flavor >= RPC_AUTH_MAXFLAVOR)
-                return -EINVAL;
-
-        ops = auth_flavors[flavor];
+        ops = rpcauth_get_authops(flavor);
         if (ops == NULL)
-                request_module("rpc-auth-%u", flavor);
-        spin_lock(&rpc_authflavor_lock);
-        ops = auth_flavors[flavor];
-        if (ops == NULL || !try_module_get(ops->owner)) {
-                spin_unlock(&rpc_authflavor_lock);
                 return -ENOENT;
-        }
-        spin_unlock(&rpc_authflavor_lock);
 
         result = -ENOENT;
         if (ops->flavor2info != NULL)
                 result = ops->flavor2info(pseudoflavor, info);
 
-        module_put(ops->owner);
+        rpcauth_put_authops(ops);
         return result;
 }
 EXPORT_SYMBOL_GPL(rpcauth_get_gssinfo);
@@ -212,15 +218,13 @@ EXPORT_SYMBOL_GPL(rpcauth_get_gssinfo);
 int
 rpcauth_list_flavors(rpc_authflavor_t *array, int size)
 {
-        rpc_authflavor_t flavor;
-        int result = 0;
+        const struct rpc_authops *ops;
+        rpc_authflavor_t flavor, pseudos[4];
+        int i, len, result = 0;
 
-        spin_lock(&rpc_authflavor_lock);
+        rcu_read_lock();
         for (flavor = 0; flavor < RPC_AUTH_MAXFLAVOR; flavor++) {
-                const struct rpc_authops *ops = auth_flavors[flavor];
-                rpc_authflavor_t pseudos[4];
-                int i, len;
-
+                ops = rcu_dereference(auth_flavors[flavor]);
                 if (result >= size) {
                         result = -ENOMEM;
                         break;
@@ -245,7 +249,7 @@ rpcauth_list_flavors(rpc_authflavor_t *array, int size)
                         array[result++] = pseudos[i];
                 }
         }
-        spin_unlock(&rpc_authflavor_lock);
+        rcu_read_unlock();
 
         dprintk("RPC:       %s returns %d\n", __func__, result);
         return result;
@@ -255,25 +259,17 @@ EXPORT_SYMBOL_GPL(rpcauth_list_flavors);
 struct rpc_auth *
 rpcauth_create(const struct rpc_auth_create_args *args, struct rpc_clnt *clnt)
 {
-        struct rpc_auth         *auth;
+        struct rpc_auth *auth = ERR_PTR(-EINVAL);
         const struct rpc_authops *ops;
-        u32                     flavor = pseudoflavor_to_flavor(args->pseudoflavor);
+        u32 flavor = pseudoflavor_to_flavor(args->pseudoflavor);
 
-        auth = ERR_PTR(-EINVAL);
-        if (flavor >= RPC_AUTH_MAXFLAVOR)
+        ops = rpcauth_get_authops(flavor);
+        if (ops == NULL)
                 goto out;
 
-        if ((ops = auth_flavors[flavor]) == NULL)
-                request_module("rpc-auth-%u", flavor);
-        spin_lock(&rpc_authflavor_lock);
-        ops = auth_flavors[flavor];
-        if (ops == NULL || !try_module_get(ops->owner)) {
-                spin_unlock(&rpc_authflavor_lock);
-                goto out;
-        }
-        spin_unlock(&rpc_authflavor_lock);
         auth = ops->create(args, clnt);
-        module_put(ops->owner);
+
+        rpcauth_put_authops(ops);
         if (IS_ERR(auth))
                 return auth;
         if (clnt->cl_auth)