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Subject[PATCH 01/16] nfs_instantiate(): prevent multiple aliases for directory inode
From: Al Viro <>

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 <>
Signed-off-by: Al Viro <>
fs/nfs/dir.c | 9 ++++++---
1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/fs/nfs/dir.c b/fs/nfs/dir.c
index 7a9c14426855..df6fd4e5b068 100644
--- 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;

@@ -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);
return 0;
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