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

1 files 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;