[PATCH v10 3/8] [RFC] Allow atomic_open() on positive dentry (O_CREAT)

[Bernd Schubert <bschubert@xxxxxxx>]

From: Miklos Szeredi <miklos@xxxxxxxxxx>

atomic_open() will do an open-by-name or create-and-open
depending on the flags.

If file was created, then the old positive dentry is obviously
stale, so it will be invalidated and a new one will be allocated.

If not created, then check whether it's the same inode (same as in
->d_revalidate()) and if not, invalidate & allocate new dentry.

This only works with O_CREAT, without O_CREAT open_last_lookups
will call into lookup_fast and then return the dentry via
finish_lookup - lookup_open is never called.
This is going to be addressed in the next commit.

Another included change is the introduction of an enum as
d_revalidate return code.

Co-developed-by: Bernd Schubert <bschubert@xxxxxxx>
Signed-off-by: Bernd Schubert <bschubert@xxxxxxx>
Signed-off-by: Miklos Szeredi <miklos@xxxxxxxxxx>
Cc: Christian Brauner <brauner@xxxxxxxxxx>
Cc: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
Cc: Dharmendra Singh <dsingh@xxxxxxx>
Cc: Amir Goldstein <amir73il@xxxxxxxxx>
Cc: linux-fsdevel@xxxxxxxxxxxxxxx
---
 fs/namei.c            | 11 ++++++-----
 include/linux/namei.h |  7 +++++++
 2 files changed, 13 insertions(+), 5 deletions(-)

diff --git a/fs/namei.c b/fs/namei.c
index 567ee547492b..ff913e6b12b4 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -860,7 +860,7 @@ static inline int d_revalidate(struct dentry *dentry, unsigned int flags)
 	if (unlikely(dentry->d_flags & DCACHE_OP_REVALIDATE))
 		return dentry->d_op->d_revalidate(dentry, flags);
 	else
-		return 1;
+		return D_REVALIDATE_VALID;
 }
 
 /**
@@ -3330,8 +3330,9 @@ static int may_o_create(struct mnt_idmap *idmap,
 }
 
 /*
- * Attempt to atomically look up, create and open a file from a negative
- * dentry.
+ * Attempt to atomically look up, create and open a file from a
+ * dentry. Unless the file system returns D_REVALIDATE_ATOMIC in ->d_revalidate,
+ * the dentry is always negative.
  *
  * Returns 0 if successful.  The file will have been created and attached to
  * @file by the filesystem calling finish_open().
@@ -3406,7 +3407,7 @@ static struct dentry *lookup_open(struct nameidata *nd, struct file *file,
 	struct inode *dir_inode = dir->d_inode;
 	int open_flag = op->open_flag;
 	struct dentry *dentry;
-	int error, create_error = 0;
+	int error = 0, create_error = 0;
 	umode_t mode = op->mode;
 	DECLARE_WAIT_QUEUE_HEAD_ONSTACK(wq);
 
@@ -3433,7 +3434,7 @@ static struct dentry *lookup_open(struct nameidata *nd, struct file *file,
 		dput(dentry);
 		dentry = NULL;
 	}
-	if (dentry->d_inode) {
+	if (dentry->d_inode && error != D_REVALIDATE_ATOMIC) {
 		/* Cached positive dentry: will open in f_op->open */
 		return dentry;
 	}
diff --git a/include/linux/namei.h b/include/linux/namei.h
index 1463cbda4888..a70e87d2b2a9 100644
--- a/include/linux/namei.h
+++ b/include/linux/namei.h
@@ -47,6 +47,13 @@ enum {LAST_NORM, LAST_ROOT, LAST_DOT, LAST_DOTDOT};
 /* LOOKUP_* flags which do scope-related checks based on the dirfd. */
 #define LOOKUP_IS_SCOPED (LOOKUP_BENEATH | LOOKUP_IN_ROOT)
 
+/* ->d_revalidate return codes */
+enum {
+	D_REVALIDATE_INVALID = 0, /* invalid dentry */
+	D_REVALIDATE_VALID   = 1, /* valid dentry */
+	D_REVALIDATE_ATOMIC =  2, /* atomic_open will revalidate */
+};
+
 extern int path_pts(struct path *path);
 
 extern int user_path_at_empty(int, const char __user *, unsigned, struct path *, int *empty);
-- 
2.39.2