[RFC PATCH] vfs: allow ->atomic_open() on positive

[Miklos Szeredi <miklos@szeredi.hu>]

Hi Al,

Do you see anything bad with allowing ->atomic_open() to take a positive dentry
and possibly invalidate it after it does the atomic LOOKUP/CREATE+OPEN?

It looks wrong not to allow optimizing away the roundtrip associated with
revalidation when we do allow optimizing away the roundtrip for the initial
lookup in the same situation.

Thanks,
Miklos


diff --git a/fs/namei.c b/fs/namei.c
index 509657fdf4f5..d35b5cbf7f64 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -3267,7 +3267,7 @@ static struct dentry *lookup_open(struct nameidata *nd, struct file *file,
 		dput(dentry);
 		dentry = NULL;
 	}
-	if (dentry->d_inode) {
+	if (dentry->d_inode && !d_atomic_open(dentry)) {
 		/* Cached positive dentry: will open in f_op->open */
 		return dentry;
 	}
diff --git a/include/linux/dcache.h b/include/linux/dcache.h
index f5bba51480b2..da681bdbc34e 100644
--- a/include/linux/dcache.h
+++ b/include/linux/dcache.h
@@ -208,6 +208,7 @@ struct dentry_operations {
 #define DCACHE_NOKEY_NAME		0x02000000 /* Encrypted name encoded without key */
 #define DCACHE_OP_REAL			0x04000000
 
+#define DCACHE_ATOMIC_OPEN		0x08000000 /* Always use ->atomic_open() to open this file */
 #define DCACHE_PAR_LOOKUP		0x10000000 /* being looked up (with parent locked shared) */
 #define DCACHE_DENTRY_CURSOR		0x20000000
 #define DCACHE_NORCU			0x40000000 /* No RCU delay for freeing */
@@ -446,6 +447,11 @@ static inline bool d_is_positive(const struct dentry *dentry)
 	return !d_is_negative(dentry);
 }
 
+static inline bool d_atomic_open(const struct dentry *dentry)
+{
+	return dentry->d_flags & DCACHE_ATOMIC_OPEN;
+}
+
 /**
  * d_really_is_negative - Determine if a dentry is really negative (ignoring fallthroughs)
  * @dentry: The dentry in question