VFS: Impose ordering on accesses of d_inode and d_flags

Impose ordering on accesses of d_inode and d_flags to avoid the need to do
this:

	if (!dentry->d_inode || d_is_negative(dentry)) {

when this:

	if (d_is_negative(dentry)) {

should suffice.

This check is especially problematic if a dentry can have its type field set
to something other than DENTRY_MISS_TYPE when d_inode is NULL (as in
unionmount).

What we really need to do is stick a write barrier between setting d_inode and
setting d_flags and a read barrier between reading d_flags and reading
d_inode.

Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

1 files changed, 3 insertions, 18 deletions

diff --git a/include/linux/dcache.h b/include/linux/dcache.h
index e83768e..df334cb 100644
--- a/include/linux/dcache.h
+++ b/include/linux/dcache.h
@@ -404,26 +404,11 @@ static inline bool d_mountpoint(const struct dentry *dentry)
 /*
  * Directory cache entry type accessor functions.
  */
-static inline void __d_set_type(struct dentry *dentry, unsigned type)
-{
-	dentry->d_flags = (dentry->d_flags & ~DCACHE_ENTRY_TYPE) | type;
-}
-
-static inline void __d_clear_type(struct dentry *dentry)
-{
-	__d_set_type(dentry, DCACHE_MISS_TYPE);
-}
-
-static inline void d_set_type(struct dentry *dentry, unsigned type)
-{
-	spin_lock(&dentry->d_lock);
-	__d_set_type(dentry, type);
-	spin_unlock(&dentry->d_lock);
-}
-
 static inline unsigned __d_entry_type(const struct dentry *dentry)
 {
-	return dentry->d_flags & DCACHE_ENTRY_TYPE;
+	unsigned type = READ_ONCE(dentry->d_flags);
+	smp_rmb();
+	return type & DCACHE_ENTRY_TYPE;
 }
 
 static inline bool d_is_miss(const struct dentry *dentry)