nfs_instantiate(): prevent multiple aliases for directory inode

Since NFS allows open-by-fhandle, we have to cope with the possibility
of mkdir vs. open-by-guessed-handle races.  A local filesystem could
decide what the inumber of the new object will be and insert a locked
inode with that inumber into icache _before_ the on-disk data structures
begin to look good and unlock it only once it has a dentry alias, so
that open-by-handle coming first would quietly fail and mkdir coming
first would have open-by-handle grab its dentry.

For NFS it's a non-starter - the icache key is server-supplied fhandle
and we do not get that until the object has been fully created on server.
We really have to deal with the possibility that open-by-handle gets
the in-core inode and attaches a dentry to it before mkdir does.

Solution: let nfs_mkdir() use d_splice_alias() to catch those.  We can
	* get an error.  Just return it to our caller.
	* get NULL - no preexisting dentry aliases, we'd just done what
d_add() would've done.  Success.
	* get a reference to preexisting alias.  In that case the alias
had been moved in place of nfs_mkdir() argument (and hashed there), while
nfs_mkdir() argument is left unhashed negative.  Which is just fine for
->mkdir() callers, all we need is to release the reference we'd got from
d_splice_alias() and report success.

Cc: Trond Myklebust <trond.myklebust@primarydata.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

diff --git a/fs/nfs/dir.c b/fs/nfs/dir.c
index 7a9c14426855309d2bb68e681b2db32321648d0c..df6fd4e5b068f4d0f1926e651d9b7ab533ce5e3e 100644 (file)
--- a/fs/nfs/dir.c
+++ b/fs/nfs/dir.c
@@ -1641,6 +1641,7 @@ int nfs_instantiate(struct dentry *dentry, struct nfs_fh *fhandle,
        struct dentry *parent = dget_parent(dentry);
        struct inode *dir = d_inode(parent);
        struct inode *inode;
+       struct dentry *d;
        int error = -EACCES;
 
        d_drop(dentry);
@@ -1662,10 +1663,12 @@ int nfs_instantiate(struct dentry *dentry, struct nfs_fh *fhandle,
                        goto out_error;
        }
        inode = nfs_fhget(dentry->d_sb, fhandle, fattr, label);
-       error = PTR_ERR(inode);
-       if (IS_ERR(inode))
+       d = d_splice_alias(inode, dentry);
+       if (IS_ERR(d)) {
+               error = PTR_ERR(d);
                goto out_error;
-       d_add(dentry, inode);
+       }
+       dput(d);
 out:
        dput(parent);
        return 0;