[added to the 4.1 stable tree] make nfs_atomic_open() call d_drop() on all ->open_context() errors.

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From: Al Viro <viro@xxxxxxxxxxxxxxxxxx>

This patch has been added to the 4.1 stable tree. If you have any
objections, please let us know.

===============

[ Upstream commit d20cb71dbf3487f24549ede1a8e2d67579b4632e ]

In "NFSv4: Move dentry instantiation into the NFSv4-specific atomic open code"
unconditional d_drop() after the ->open_context() had been removed.  It had
been correct for success cases (there ->open_context() itself had been doing
dcache manipulations), but not for error ones.  Only one of those (ENOENT)
got a compensatory d_drop() added in that commit, but in fact it should've
been done for all errors.  As it is, the case of O_CREAT non-exclusive open
on a hashed negative dentry racing with e.g. symlink creation from another
client ended up with ->open_context() getting an error and proceeding to
call nfs_lookup().  On a hashed dentry, which would've instantly triggered
BUG_ON() in d_materialise_unique() (or, these days, its equivalent in
d_splice_alias()).

Cc: stable@xxxxxxxxxxxxxxx # v3.10+
Tested-by: Oleg Drokin <green@xxxxxxxxxxxxxx>
Signed-off-by: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
Signed-off-by: Trond Myklebust <trond.myklebust@xxxxxxxxxxxxxxx>
Signed-off-by: Anna Schumaker <Anna.Schumaker@xxxxxxxxxx>
Signed-off-by: Sasha Levin <sasha.levin@xxxxxxxxxx>
---
 fs/nfs/dir.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/nfs/dir.c b/fs/nfs/dir.c
index b2c8b31..aadb4af 100644
--- a/fs/nfs/dir.c
+++ b/fs/nfs/dir.c
@@ -1542,9 +1542,9 @@ int nfs_atomic_open(struct inode *dir, struct dentry *dentry,
 		err = PTR_ERR(inode);
 		trace_nfs_atomic_open_exit(dir, ctx, open_flags, err);
 		put_nfs_open_context(ctx);
+		d_drop(dentry);
 		switch (err) {
 		case -ENOENT:
-			d_drop(dentry);
 			d_add(dentry, NULL);
 			nfs_set_verifier(dentry, nfs_save_change_attribute(dir));
 			break;
-- 
2.5.0

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