Allow sharing external names after __d_move()

* external dentry names get a small structure prepended to them
(struct external_name).
* it contains an atomic refcount, matching the number of struct dentry
instances that have ->d_name.name pointing to that external name.  The
first thing free_dentry() does is decrementing refcount of external name,
so the instances that are between the call of free_dentry() and
RCU-delayed actual freeing do not contribute.
* __d_move(x, y, false) makes the name of x equal to the name of y,
external or not.  If y has an external name, extra reference is grabbed
and put into x->d_name.name.  If x used to have an external name, the
reference to the old name is dropped and, should it reach zero, freeing
is scheduled via kfree_rcu().
* free_dentry() in dentry with external name decrements the refcount of
that name and, should it reach zero, does RCU-delayed call that will
free both the dentry and external name.  Otherwise it does what it
used to do, except that __d_free() doesn't even look at ->d_name.name;
it simply frees the dentry.

All non-RCU accesses to dentry external name are safe wrt freeing since they
all should happen before free_dentry() is called.  RCU accesses might run
into a dentry seen by free_dentry() or into an old name that got already
dropped by __d_move(); however, in both cases dentry must have been
alive and refer to that name at some point after we'd done rcu_read_lock(),
which means that any freeing must be still pending.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

diff --git a/fs/dcache.c b/fs/dcache.c
index e7484f9..4858d2e 100644 (file)
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -235,18 +235,44 @@ static inline int dentry_cmp(const struct dentry *dentry, const unsigned char *c
        return dentry_string_cmp(cs, ct, tcount);
 }
 
+struct external_name {
+       union {
+               atomic_t count;
+               struct rcu_head head;
+       } u;
+       unsigned char name[];
+};
+
+static inline struct external_name *external_name(struct dentry *dentry)
+{
+       return container_of(dentry->d_name.name, struct external_name, name[0]);
+}
+
 static void __d_free(struct rcu_head *head)
 {
        struct dentry *dentry = container_of(head, struct dentry, d_u.d_rcu);
 
        WARN_ON(!hlist_unhashed(&dentry->d_alias));
-       if (dname_external(dentry))
-               kfree(dentry->d_name.name);
+       kmem_cache_free(dentry_cache, dentry); 
+}
+
+static void __d_free_external(struct rcu_head *head)
+{
+       struct dentry *dentry = container_of(head, struct dentry, d_u.d_rcu);
+       WARN_ON(!hlist_unhashed(&dentry->d_alias));
+       kfree(external_name(dentry));
        kmem_cache_free(dentry_cache, dentry); 
 }
 
 static void dentry_free(struct dentry *dentry)
 {
+       if (unlikely(dname_external(dentry))) {
+               struct external_name *p = external_name(dentry);
+               if (likely(atomic_dec_and_test(&p->u.count))) {
+                       call_rcu(&dentry->d_u.d_rcu, __d_free_external);
+                       return;
+               }
+       }
        /* if dentry was never visible to RCU, immediate free is OK */
        if (!(dentry->d_flags & DCACHE_RCUACCESS))
                __d_free(&dentry->d_u.d_rcu);
@@ -1438,11 +1464,14 @@ struct dentry *__d_alloc(struct super_block *sb, const struct qstr *name)
         */
        dentry->d_iname[DNAME_INLINE_LEN-1] = 0;
        if (name->len > DNAME_INLINE_LEN-1) {
-               dname = kmalloc(name->len + 1, GFP_KERNEL);
-               if (!dname) {
+               size_t size = offsetof(struct external_name, name[1]);
+               struct external_name *p = kmalloc(size + name->len, GFP_KERNEL);
+               if (!p) {
                        kmem_cache_free(dentry_cache, dentry); 
                        return NULL;
                }
+               atomic_set(&p->u.count, 1);
+               dname = p->name;
        } else  {
                dname = dentry->d_iname;
        }       
@@ -2372,11 +2401,10 @@ void dentry_update_name_case(struct dentry *dentry, struct qstr *name)
 }
 EXPORT_SYMBOL(dentry_update_name_case);
 
-static void switch_names(struct dentry *dentry, struct dentry *target,
-                        bool exchange)
+static void swap_names(struct dentry *dentry, struct dentry *target)
 {
-       if (dname_external(target)) {
-               if (dname_external(dentry)) {
+       if (unlikely(dname_external(target))) {
+               if (unlikely(dname_external(dentry))) {
                        /*
                         * Both external: swap the pointers
                         */
@@ -2392,7 +2420,7 @@ static void switch_names(struct dentry *dentry, struct dentry *target,
                        target->d_name.name = target->d_iname;
                }
        } else {
-               if (dname_external(dentry)) {
+               if (unlikely(dname_external(dentry))) {
                        /*
                         * dentry:external, target:internal.  Give dentry's
                         * storage to target and make dentry internal
@@ -2407,12 +2435,6 @@ static void switch_names(struct dentry *dentry, struct dentry *target,
                         */
                        unsigned int i;
                        BUILD_BUG_ON(!IS_ALIGNED(DNAME_INLINE_LEN, sizeof(long)));
-                       if (!exchange) {
-                               memcpy(dentry->d_iname, target->d_name.name,
-                                               target->d_name.len + 1);
-                               dentry->d_name.hash_len = target->d_name.hash_len;
-                               return;
-                       }
                        for (i = 0; i < DNAME_INLINE_LEN / sizeof(long); i++) {
                                swap(((long *) &dentry->d_iname)[i],
                                     ((long *) &target->d_iname)[i]);
@@ -2422,6 +2444,24 @@ static void switch_names(struct dentry *dentry, struct dentry *target,
        swap(dentry->d_name.hash_len, target->d_name.hash_len);
 }
 
+static void copy_name(struct dentry *dentry, struct dentry *target)
+{
+       struct external_name *old_name = NULL;
+       if (unlikely(dname_external(dentry)))
+               old_name = external_name(dentry);
+       if (unlikely(dname_external(target))) {
+               atomic_inc(&external_name(target)->u.count);
+               dentry->d_name = target->d_name;
+       } else {
+               memcpy(dentry->d_iname, target->d_name.name,
+                               target->d_name.len + 1);
+               dentry->d_name.name = dentry->d_iname;
+               dentry->d_name.hash_len = target->d_name.hash_len;
+       }
+       if (old_name && likely(atomic_dec_and_test(&old_name->u.count)))
+               kfree_rcu(old_name, u.head);
+}
+
 static void dentry_lock_for_move(struct dentry *dentry, struct dentry *target)
 {
        /*
@@ -2518,7 +2558,10 @@ static void __d_move(struct dentry *dentry, struct dentry *target,
        }
 
        /* Switch the names.. */
-       switch_names(dentry, target, exchange);
+       if (exchange)
+               swap_names(dentry, target);
+       else
+               copy_name(dentry, target);
 
        /* ... and switch them in the tree */
        if (IS_ROOT(dentry)) {