sunrpc/auth: allow lockless (rcu) lookup of credential cache.

The new flag RPCAUTH_LOOKUP_RCU to credential lookup avoids locking,
does not take a reference on the returned credential, and returns
-ECHILD if a simple lookup was not possible.

The returned value can only be used within an rcu_read_lock protected
region.

The main user of this is the new rpc_lookup_cred_nonblock() which
returns a pointer to the current credential which is only rcu-safe (no
ref-count held), and might return -ECHILD if allocation was required.

Signed-off-by: NeilBrown <neilb@suse.de>
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>

3 files changed, 23 insertions, 2 deletions

diff --git a/net/sunrpc/auth.c b/net/sunrpc/auth.c
index 360decdddc78..24fcbd23ae6c 100644
--- a/net/sunrpc/auth.c
+++ b/net/sunrpc/auth.c
@@ -557,6 +557,12 @@ rpcauth_lookup_credcache(struct rpc_auth *auth, struct auth_cred * acred,
         hlist_for_each_entry_rcu(entry, &cache->hashtable[nr], cr_hash) {
                 if (!entry->cr_ops->crmatch(acred, entry, flags))
                         continue;
+                if (flags & RPCAUTH_LOOKUP_RCU) {
+                        if (test_bit(RPCAUTH_CRED_HASHED, &entry->cr_flags) &&
+                            !test_bit(RPCAUTH_CRED_NEW, &entry->cr_flags))
+                                cred = entry;
+                        break;
+                }
                 spin_lock(&cache->lock);
                 if (test_bit(RPCAUTH_CRED_HASHED, &entry->cr_flags) == 0) {
                         spin_unlock(&cache->lock);
@@ -571,6 +577,9 @@ rpcauth_lookup_credcache(struct rpc_auth *auth, struct auth_cred * acred,
         if (cred != NULL)
                 goto found;
 
+        if (flags & RPCAUTH_LOOKUP_RCU)
+                return ERR_PTR(-ECHILD);
+
         new = auth->au_ops->crcreate(auth, acred, flags);
         if (IS_ERR(new)) {
                 cred = new;
@@ -621,10 +630,14 @@ rpcauth_lookupcred(struct rpc_auth *auth, int flags)
         memset(&acred, 0, sizeof(acred));
         acred.uid = cred->fsuid;
         acred.gid = cred->fsgid;
-        acred.group_info = get_group_info(((struct cred *)cred)->group_info);
+        if (flags & RPCAUTH_LOOKUP_RCU)
+                acred.group_info = rcu_dereference(cred->group_info);
+        else
+                acred.group_info = get_group_info(((struct cred *)cred)->group_info);
 
         ret = auth->au_ops->lookup_cred(auth, &acred, flags);
-        put_group_info(acred.group_info);
+        if (!(flags & RPCAUTH_LOOKUP_RCU))
+                put_group_info(acred.group_info);
         return ret;
 }
 EXPORT_SYMBOL_GPL(rpcauth_lookupcred);
diff --git a/net/sunrpc/auth_generic.c b/net/sunrpc/auth_generic.c
index ed04869b2d4f..6f6b829c9e8e 100644
--- a/net/sunrpc/auth_generic.c
+++ b/net/sunrpc/auth_generic.c
@@ -38,6 +38,12 @@ struct rpc_cred *rpc_lookup_cred(void)
 }
 EXPORT_SYMBOL_GPL(rpc_lookup_cred);
 
+struct rpc_cred *rpc_lookup_cred_nonblock(void)
+{
+        return rpcauth_lookupcred(&generic_auth, RPCAUTH_LOOKUP_RCU);
+}
+EXPORT_SYMBOL_GPL(rpc_lookup_cred_nonblock);
+
 /*
  * Public call interface for looking up machine creds.
  */
diff --git a/net/sunrpc/auth_null.c b/net/sunrpc/auth_null.c
index f0ebe07978a2..712c123e04e9 100644
--- a/net/sunrpc/auth_null.c
+++ b/net/sunrpc/auth_null.c
@@ -35,6 +35,8 @@ nul_destroy(struct rpc_auth *auth)
 static struct rpc_cred *
 nul_lookup_cred(struct rpc_auth *auth, struct auth_cred *acred, int flags)
 {
+        if (flags & RPCAUTH_LOOKUP_RCU)
+                return &null_cred;
         return get_rpccred(&null_cred);
 }