Commit 8d85b484 authored by Al Viro's avatar Al Viro
Browse files

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: default avatarAl Viro <viro@zeniv.linux.org.uk>
parent 6d13f694

fs/dcache.c

+59 −16
Original line number Diff line number Diff line
@@ -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)) {

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