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@xxxxxxx>
---
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 3c2b36acf291..ac958f29e558 100644
--- a/fs/nfs/dir.c
+++ b/fs/nfs/dir.c
@@ -2288,6 +2288,10 @@ 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;
@@ -2364,15 +2368,23 @@ 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;
--
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