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>

diff --git a/fs/nfs/dir.c b/fs/nfs/dir.c
index ea12e58dfd853241947584535ed25918f78292d6..8a3c36984fc48d2558c096b3c2491127e886a80a 100644 (file)
--- 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;