From f3324a2a94c229831cfd42d871902cd4a9bd5e0f Mon Sep 17 00:00:00 2001 From: NeilBrown Date: Mon, 14 Jul 2014 11:28:20 +1000 Subject: [PATCH] NFS: support RCU_WALK in nfs_permission() nfs_permission makes two calls which are not always safe in RCU_WALK, rpc_lookup_cred and nfs_do_access. The second can easily be made rcu-safe by aborting with -ECHILD before making the RPC call. The former can be made rcu-safe by calling rpc_lookup_cred_nonblock() instead. As this will almost always succeed, we use it even when RCU_WALK isn't being used as it still saves some spinlocks in a common case. We only fall back to rpc_lookup_cred() if rpc_lookup_cred_nonblock() fails and MAY_NOT_BLOCK isn't set. This optimisation (always trying rpc_lookup_cred_nonblock()) is particularly important when a security module is active. In that case inode_permission() may return -ECHILD from security_inode_permission() even though ->permission() succeeded in RCU_WALK mode. This leads to may_lookup() retrying inode_permission after performing unlazy_walk(). The spinlock that rpc_lookup_cred() takes is often more expensive than anything security_inode_permission() does, so that spinlock becomes the main bottleneck. Signed-off-by: NeilBrown Signed-off-by: Trond Myklebust --- fs/nfs/dir.c | 28 ++++++++++++++++++++-------- 1 file changed, 20 insertions(+), 8 deletions(-) diff --git a/fs/nfs/dir.c b/fs/nfs/dir.c index ea12e58dfd85..8a3c36984fc4 100644 --- a/fs/nfs/dir.c +++ b/fs/nfs/dir.c @@ -2316,6 +2316,10 @@ static int nfs_do_access(struct inode *inode, struct rpc_cred *cred, int mask) if (status == 0) goto out_cached; + status = -ECHILD; + if (mask & MAY_NOT_BLOCK) + goto out; + /* Be clever: ask server to check for all possible rights */ cache.mask = MAY_EXEC | MAY_WRITE | MAY_READ; cache.cred = cred; @@ -2392,15 +2396,23 @@ force_lookup: if (!NFS_PROTO(inode)->access) goto out_notsup; - if (mask & MAY_NOT_BLOCK) - return -ECHILD; - - cred = rpc_lookup_cred(); - if (!IS_ERR(cred)) { - res = nfs_do_access(inode, cred, mask); - put_rpccred(cred); - } else + /* Always try fast lookups first */ + rcu_read_lock(); + cred = rpc_lookup_cred_nonblock(); + if (!IS_ERR(cred)) + res = nfs_do_access(inode, cred, mask|MAY_NOT_BLOCK); + else res = PTR_ERR(cred); + rcu_read_unlock(); + if (res == -ECHILD && !(mask & MAY_NOT_BLOCK)) { + /* Fast lookup failed, try the slow way */ + cred = rpc_lookup_cred(); + if (!IS_ERR(cred)) { + res = nfs_do_access(inode, cred, mask); + put_rpccred(cred); + } else + res = PTR_ERR(cred); + } out: if (!res && (mask & MAY_EXEC) && !execute_ok(inode)) res = -EACCES;