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>

diff --git a/fs/dcache.c b/fs/dcache.c
index d99736a..656ce52 100644 (file)
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -269,6 +269,41 @@ static inline int dname_external(const struct dentry *dentry)
        return dentry->d_name.name != dentry->d_iname;
 }
 
+/*
+ * Make sure other CPUs see the inode attached before the type is set.
+ */
+static inline void __d_set_inode_and_type(struct dentry *dentry,
+                                         struct inode *inode,
+                                         unsigned type_flags)
+{
+       unsigned flags;
+
+       dentry->d_inode = inode;
+       smp_wmb();
+       flags = READ_ONCE(dentry->d_flags);
+       flags &= ~(DCACHE_ENTRY_TYPE | DCACHE_FALLTHRU);
+       flags |= type_flags;
+       WRITE_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 = READ_ONCE(dentry->d_flags);
+
+       flags &= ~(DCACHE_ENTRY_TYPE | DCACHE_FALLTHRU);
+       WRITE_ONCE(dentry->d_flags, flags);
+       smp_wmb();
+       dentry->d_inode = NULL;
+}
+
 static void dentry_free(struct dentry *dentry)
 {
        WARN_ON(!hlist_unhashed(&dentry->d_u.d_alias));
@@ -311,7 +346,7 @@ static void dentry_iput(struct dentry * dentry)
 {
        struct inode *inode = dentry->d_inode;
        if (inode) {
-               dentry->d_inode = NULL;
+               __d_clear_type_and_inode(dentry);
                hlist_del_init(&dentry->d_u.d_alias);
                spin_unlock(&dentry->d_lock);
                spin_unlock(&inode->i_lock);
@@ -335,8 +370,7 @@ static void dentry_unlink_inode(struct dentry * dentry)
        __releases(dentry->d_inode->i_lock)
 {
        struct inode *inode = dentry->d_inode;
-       __d_clear_type(dentry);
-       dentry->d_inode = NULL;
+       __d_clear_type_and_inode(dentry);
        hlist_del_init(&dentry->d_u.d_alias);
        dentry_rcuwalk_barrier(dentry);
        spin_unlock(&dentry->d_lock);
@@ -1715,11 +1749,9 @@ static void __d_instantiate(struct dentry *dentry, struct inode *inode)
        unsigned add_flags = d_flags_for_inode(inode);
 
        spin_lock(&dentry->d_lock);
-       dentry->d_flags &= ~(DCACHE_ENTRY_TYPE | DCACHE_FALLTHRU);
-       dentry->d_flags |= add_flags;
        if (inode)
                hlist_add_head(&dentry->d_u.d_alias, &inode->i_dentry);
-       dentry->d_inode = inode;
+       __d_set_inode_and_type(dentry, inode, add_flags);
        dentry_rcuwalk_barrier(dentry);
        spin_unlock(&dentry->d_lock);
        fsnotify_d_instantiate(dentry, inode);
@@ -1937,8 +1969,7 @@ static struct dentry *__d_obtain_alias(struct inode *inode, int disconnected)
                add_flags |= DCACHE_DISCONNECTED;
 
        spin_lock(&tmp->d_lock);
-       tmp->d_inode = inode;
-       tmp->d_flags |= add_flags;
+       __d_set_inode_and_type(tmp, inode, add_flags);
        hlist_add_head(&tmp->d_u.d_alias, &inode->i_dentry);
        hlist_bl_lock(&tmp->d_sb->s_anon);
        hlist_bl_add_head(&tmp->d_hash, &tmp->d_sb->s_anon);

diff --git a/include/linux/dcache.h b/include/linux/dcache.h
index e83768e..df334cb 100644 (file)
--- 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)