[PATCH 3.16 318/366] use ->d_seq to get coherency between ->d_inode and ->d_flags

[Ben Hutchings <ben@xxxxxxxxxxxxxxx>]

3.16.61-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Al Viro <viro@xxxxxxxxxxxxxxxxxx>

commit a528aca7f359f4b0b1d72ae406097e491a5ba9ea upstream.

Games with ordering and barriers are way too brittle.  Just
bump ->d_seq before and after updating ->d_inode and ->d_flags
type bits, so that verifying ->d_seq would guarantee they are
coherent.

Signed-off-by: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
[bwh: Backported to 3.16: adjust context]
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
 fs/dcache.c            | 20 +++++---------------
 include/linux/dcache.h |  4 +---
 2 files changed, 6 insertions(+), 18 deletions(-)

--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -291,28 +291,18 @@ static inline void __d_set_inode_and_typ
 	unsigned flags;
 
 	dentry->d_inode = inode;
-	smp_wmb();
 	flags = ACCESS_ONCE(dentry->d_flags);
 	flags &= ~DCACHE_ENTRY_TYPE;
 	flags |= type_flags;
 	ACCESS_ONCE(dentry->d_flags) = flags;
 }
 
-/*
- * Ideally, we want to make sure that other CPUs see the flags cleared before
- * the inode is detached, but this is really a violation of RCU principles
- * since the ordering suggests we should always set inode before flags.
- *
- * We should instead replace or discard the entire dentry - but that sucks
- * performancewise on mass deletion/rename.
- */
 static inline void __d_clear_type_and_inode(struct dentry *dentry)
 {
 	unsigned flags = ACCESS_ONCE(dentry->d_flags);
 
 	flags &= ~DCACHE_ENTRY_TYPE;
 	ACCESS_ONCE(dentry->d_flags) = flags;
-	smp_wmb();
 	dentry->d_inode = NULL;
 }
 
@@ -376,9 +366,11 @@ static void dentry_unlink_inode(struct d
 	__releases(dentry->d_inode->i_lock)
 {
 	struct inode *inode = dentry->d_inode;
+
+	raw_write_seqcount_begin(&dentry->d_seq);
 	__d_clear_type_and_inode(dentry);
 	hlist_del_init(&dentry->d_u.d_alias);
-	dentry_rcuwalk_barrier(dentry);
+	raw_write_seqcount_end(&dentry->d_seq);
 	spin_unlock(&dentry->d_lock);
 	spin_unlock(&inode->i_lock);
 	if (!inode->i_nlink)
@@ -1680,8 +1672,9 @@ static void __d_instantiate(struct dentr
 	spin_lock(&dentry->d_lock);
 	if (inode)
 		hlist_add_head(&dentry->d_u.d_alias, &inode->i_dentry);
+	raw_write_seqcount_begin(&dentry->d_seq);
 	__d_set_inode_and_type(dentry, inode, add_flags);
-	dentry_rcuwalk_barrier(dentry);
+	raw_write_seqcount_end(&dentry->d_seq);
 	spin_unlock(&dentry->d_lock);
 	fsnotify_d_instantiate(dentry, inode);
 }
--- a/include/linux/dcache.h
+++ b/include/linux/dcache.h
@@ -413,9 +413,7 @@ static inline bool d_mountpoint(const st
  */
 static inline unsigned __d_entry_type(const struct dentry *dentry)
 {
-	unsigned type = ACCESS_ONCE(dentry->d_flags);
-	smp_rmb();
-	return type & DCACHE_ENTRY_TYPE;
+	return dentry->d_flags & DCACHE_ENTRY_TYPE;
 }
 
 static inline bool d_can_lookup(const struct dentry *dentry)