Commit 95a4ccbb authored by Al Viro's avatar Al Viro
Browse files

dissolve external_name.u into separate members 



... and document the constraints on the layout.  Kept separate from
the previous commit to keep the noise separate from actual changes.
The reason for explicit __aligned() on ->name[] rather than relying
upon the alignment of the previous field is that the previous iteration
of that commit tried to save 4 bytes on 64bit by eliminating a hole
in there, which broke the assumptions in dentry_string_cmp().
Better spell it out and avoid the temptation for the future...

Reviewed-by: default avatarJeff Layton <jlayton@kernel.org>
Reviewed-by: default avatarJan Kara <jack@suse.cz>
Signed-off-by: default avatarAl Viro <viro@zeniv.linux.org.uk>
parent 1c9be84a

fs/dcache.c

+17 −13
Original line number Diff line number Diff line
@@ -295,12 +295,16 @@ static inline int dentry_cmp(const struct dentry *dentry, const unsigned char *c 
	return dentry_string_cmp(cs, ct, tcount);

}



/*

 * long names are allocated separately from dentry and never modified.

 * Refcounted, freeing is RCU-delayed.  See take_dentry_name_snapshot()

 * for the reason why ->count and ->head can't be combined into a union.

 * dentry_string_cmp() relies upon ->name[] being word-aligned.

 */

struct external_name {

	struct {

		atomic_t count;		// ->count and ->head can't be combined

		struct rcu_head head;	// see take_dentry_name_snapshot()

	} u;

	unsigned char name[];

	atomic_t count;

	struct rcu_head head;

	unsigned char name[] __aligned(sizeof(unsigned long));

};



static inline struct external_name *external_name(struct dentry *dentry)
@@ -344,7 +348,7 @@ void take_dentry_name_snapshot(struct name_snapshot *name, struct dentry *dentry 
		struct external_name *p;

		p = container_of(s, struct external_name, name[0]);

		// get a valid reference

		if (unlikely(!atomic_inc_not_zero(&p->u.count)))

		if (unlikely(!atomic_inc_not_zero(&p->count)))

			goto retry;

		name->name.name = s;

	}
@@ -361,8 +365,8 @@ void release_dentry_name_snapshot(struct name_snapshot *name) 
	if (unlikely(name->name.name != name->inline_name.string)) {

		struct external_name *p;

		p = container_of(name->name.name, struct external_name, name[0]);

		if (unlikely(atomic_dec_and_test(&p->u.count)))

			kfree_rcu(p, u.head);

		if (unlikely(atomic_dec_and_test(&p->count)))

			kfree_rcu(p, head);

	}

}

EXPORT_SYMBOL(release_dentry_name_snapshot);
@@ -400,7 +404,7 @@ static void dentry_free(struct dentry *dentry) 
	WARN_ON(!hlist_unhashed(&dentry->d_u.d_alias));

	if (unlikely(dname_external(dentry))) {

		struct external_name *p = external_name(dentry);

		if (likely(atomic_dec_and_test(&p->u.count))) {

		if (likely(atomic_dec_and_test(&p->count))) {

			call_rcu(&dentry->d_u.d_rcu, __d_free_external);

			return;

		}
@@ -1681,7 +1685,7 @@ static struct dentry *__d_alloc(struct super_block *sb, const struct qstr *name) 
			kmem_cache_free(dentry_cache, dentry); 

			return NULL;

		}

		atomic_set(&p->u.count, 1);

		atomic_set(&p->count, 1);

		dname = p->name;

	} else  {

		dname = dentry->d_shortname.string;
@@ -2774,15 +2778,15 @@ static void copy_name(struct dentry *dentry, struct dentry *target) 
	if (unlikely(dname_external(dentry)))

		old_name = external_name(dentry);

	if (unlikely(dname_external(target))) {

		atomic_inc(&external_name(target)->u.count);

		atomic_inc(&external_name(target)->count);

		dentry->d_name = target->d_name;

	} else {

		dentry->d_shortname = target->d_shortname;

		dentry->d_name.name = dentry->d_shortname.string;

		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);

	if (old_name && likely(atomic_dec_and_test(&old_name->count)))

		kfree_rcu(old_name, head);

}



/*