use ->d_seq to get coherency between ->d_inode and ->d_flags

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.

Cc: stable@vger.kernel.org # v3.13+
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

diff --git a/fs/dcache.c b/fs/dcache.c
index 92d5140..2398f9f 100644 (file)
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -269,9 +269,6 @@ 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)
@@ -279,28 +276,18 @@ static inline void __d_set_inode_and_type(struct dentry *dentry,
        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;
 }
 
@@ -370,9 +357,11 @@ static void dentry_unlink_inode(struct dentry * dentry)
        __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_invalidate(dentry);
+       raw_write_seqcount_end(&dentry->d_seq);
        spin_unlock(&dentry->d_lock);
        spin_unlock(&inode->i_lock);
        if (!inode->i_nlink)
@@ -1758,8 +1747,9 @@ static void __d_instantiate(struct dentry *dentry, struct inode *inode)
        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_invalidate(dentry);
+       raw_write_seqcount_end(&dentry->d_seq);
        spin_unlock(&dentry->d_lock);
        fsnotify_d_instantiate(dentry, inode);
 }

diff --git a/include/linux/dcache.h b/include/linux/dcache.h
index 7781ce1..c4b5f4b 100644 (file)
--- a/include/linux/dcache.h
+++ b/include/linux/dcache.h
@@ -409,9 +409,7 @@ static inline bool d_mountpoint(const struct dentry *dentry)
  */
 static inline unsigned __d_entry_type(const struct dentry *dentry)
 {
-       unsigned type = READ_ONCE(dentry->d_flags);
-       smp_rmb();
-       return type & DCACHE_ENTRY_TYPE;
+       return dentry->d_flags & DCACHE_ENTRY_TYPE;
 }
 
 static inline bool d_is_miss(const struct dentry *dentry)