NFS: add atomic_open for NFSv3 to handle O_TRUNC correctly.

With two clients, each with NFSv3 mounts of the same directory, the sequence:

   client1            client2
  ls -l afile
                      echo hello there > afile
  echo HELLO > afile
  cat afile

will show
   HELLO
   there

because the O_TRUNC requested in the final 'echo' doesn't take effect.
This is because the "Negative dentry, just create a file" section in
lookup_open() assumes that the file *does* get created since the dentry
was negative, so it sets FMODE_CREATED, and this causes do_open() to
clear O_TRUNC and so the file doesn't get truncated.

Even mounting with -o lookupcache=none does not help as
nfs_neg_need_reval() always returns false if LOOKUP_CREATE is set.

This patch fixes the problem by providing an atomic_open inode operation
for NFSv3 (and v2).  The code is largely the code from the branch in
lookup_open() when atomic_open is not provided.  The significant change
is that the O_TRUNC flag is passed a new nfs_do_create() which add
'trunc' handling to nfs_create().

With this change we also optimise away an unnecessary LOOKUP before the
file is created.

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

1 files changed, 1 insertions, 0 deletions

diff --git a/fs/nfs/nfs3proc.c b/fs/nfs/nfs3proc.c
index cbbe3f0193b8..74bda639a7cf 100644
--- a/fs/nfs/nfs3proc.c
+++ b/fs/nfs/nfs3proc.c
@@ -986,6 +986,7 @@ static int nfs3_have_delegation(struct inode *inode, fmode_t flags)
 
 static const struct inode_operations nfs3_dir_inode_operations = {
 	.create		= nfs_create,
+	.atomic_open	= nfs_atomic_open_v23,
 	.lookup		= nfs_lookup,
 	.link		= nfs_link,
 	.unlink		= nfs_unlink,