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
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -235,18 +235,44 @@
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 @@
*/
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 @@
}
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 @@
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 @@
*/
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 @@
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 @@
}
/* 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)) {