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, 3 insertions, 0 deletions

diff --git a/include/linux/nfs_fs.h b/include/linux/nfs_fs.h
index d59116ac8209..039898d70954 100644
--- a/include/linux/nfs_fs.h
+++ b/include/linux/nfs_fs.h
@@ -561,6 +561,9 @@ extern int nfs_may_open(struct inode *inode, const struct cred *cred, int openfl
 extern void nfs_access_zap_cache(struct inode *inode);
 extern int nfs_access_get_cached(struct inode *inode, const struct cred *cred,
 				 u32 *mask, bool may_block);
+extern int nfs_atomic_open_v23(struct inode *dir, struct dentry *dentry,
+			       struct file *file, unsigned int open_flags,
+			       umode_t mode);
 
 /*
  * linux/fs/nfs/symlink.c