sysfs: use singly-linked list for sysfs_dirent tree

Make sysfs_dirent use singly linked list for its tree structure.
sysfs_link_sibling() and sysfs_unlink_sibling() functions are added to
handle simpler cases.  It adds some complexity and cpu cycle overhead
but reduced memory footprint is worthwhile on big machines.

This change reduces the sizeof sysfs_dirent from 104 to 88 on 64bit
and from 60 to 52 on 32bit.

Signed-off-by: Tejun Heo <htejun@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

4 files changed, 111 insertions, 54 deletions

diff --git a/fs/sysfs/dir.c b/fs/sysfs/dir.c
index 40596a0eee52..b4074adbab01 100644
--- a/fs/sysfs/dir.c
+++ b/fs/sysfs/dir.c
@@ -22,6 +22,48 @@ static spinlock_t sysfs_ino_lock = SPIN_LOCK_UNLOCKED;
 static DEFINE_IDA(sysfs_ino_ida);
 
 /**
+ *      sysfs_link_sibling - link sysfs_dirent into sibling list
+ *      @sd: sysfs_dirent of interest
+ *
+ *      Link @sd into its sibling list which starts from
+ *      sd->s_parent->s_children.
+ *
+ *      Locking:
+ *      mutex_lock(sd->s_parent->dentry->d_inode->i_mutex)
+ */
+static void sysfs_link_sibling(struct sysfs_dirent *sd)
+{
+        struct sysfs_dirent *parent_sd = sd->s_parent;
+
+        BUG_ON(sd->s_sibling);
+        sd->s_sibling = parent_sd->s_children;
+        parent_sd->s_children = sd;
+}
+
+/**
+ *      sysfs_unlink_sibling - unlink sysfs_dirent from sibling list
+ *      @sd: sysfs_dirent of interest
+ *
+ *      Unlink @sd from its sibling list which starts from
+ *      sd->s_parent->s_children.
+ *
+ *      Locking:
+ *      mutex_lock(sd->s_parent->dentry->d_inode->i_mutex)
+ */
+static void sysfs_unlink_sibling(struct sysfs_dirent *sd)
+{
+        struct sysfs_dirent **pos;
+
+        for (pos = &sd->s_parent->s_children; *pos; pos = &(*pos)->s_sibling) {
+                if (*pos == sd) {
+                        *pos = sd->s_sibling;
+                        sd->s_sibling = NULL;
+                        break;
+                }
+        }
+}
+
+/**
  *      sysfs_get_active - get an active reference to sysfs_dirent
  *      @sd: sysfs_dirent to get an active reference to
  *
@@ -73,9 +115,9 @@ void sysfs_put_active(struct sysfs_dirent *sd)
                 return;
 
         /* atomic_dec_return() is a mb(), we'll always see the updated
-         * sd->s_sibling.next.
+         * sd->s_sibling.
          */
-        cmpl = (void *)sd->s_sibling.next;
+        cmpl = (void *)sd->s_sibling;
         complete(cmpl);
 }
 
@@ -129,18 +171,18 @@ void sysfs_deactivate(struct sysfs_dirent *sd)
         DECLARE_COMPLETION_ONSTACK(wait);
         int v;
 
-        BUG_ON(!list_empty(&sd->s_sibling));
-        sd->s_sibling.next = (void *)&wait;
+        BUG_ON(sd->s_sibling);
+        sd->s_sibling = (void *)&wait;
 
         /* atomic_add_return() is a mb(), put_active() will always see
-         * the updated sd->s_sibling.next.
+         * the updated sd->s_sibling.
          */
         v = atomic_add_return(SD_DEACTIVATED_BIAS, &sd->s_active);
 
         if (v != SD_DEACTIVATED_BIAS)
                 wait_for_completion(&wait);
 
-        INIT_LIST_HEAD(&sd->s_sibling);
+        sd->s_sibling = NULL;
 }
 
 static int sysfs_alloc_ino(ino_t *pino)
@@ -237,8 +279,6 @@ struct sysfs_dirent *sysfs_new_dirent(const char *name, umode_t mode, int type)
         atomic_set(&sd->s_count, 1);
         atomic_set(&sd->s_active, 0);
         atomic_set(&sd->s_event, 1);
-        INIT_LIST_HEAD(&sd->s_children);
-        INIT_LIST_HEAD(&sd->s_sibling);
 
         sd->s_name = name;
         sd->s_mode = mode;
@@ -273,7 +313,7 @@ void sysfs_attach_dirent(struct sysfs_dirent *sd,
 
         if (parent_sd) {
                 sd->s_parent = sysfs_get(parent_sd);
-                list_add(&sd->s_sibling, &parent_sd->s_children);
+                sysfs_link_sibling(sd);
         }
 }
 
@@ -289,7 +329,7 @@ int sysfs_dirent_exist(struct sysfs_dirent *parent_sd,
 {
         struct sysfs_dirent * sd;
 
-        list_for_each_entry(sd, &parent_sd->s_children, s_sibling) {
+        for (sd = parent_sd->s_children; sd; sd = sd->s_sibling) {
                 if (sd->s_type) {
                         if (strcmp(sd->s_name, new))
                                 continue;
@@ -409,7 +449,7 @@ static struct dentry * sysfs_lookup(struct inode *dir, struct dentry *dentry,
         struct inode *inode;
         int found = 0;
 
-        list_for_each_entry(sd, &parent_sd->s_children, s_sibling) {
+        for (sd = parent_sd->s_children; sd; sd = sd->s_sibling) {
                 if ((sd->s_type & SYSFS_NOT_PINNED) &&
                     !strcmp(sd->s_name, dentry->d_name.name)) {
                         found = 1;
@@ -458,7 +498,7 @@ static void remove_dir(struct dentry * d)
 
         mutex_lock(&parent->d_inode->i_mutex);
 
-        list_del_init(&sd->s_sibling);
+        sysfs_unlink_sibling(sd);
 
         pr_debug(" o %s removing done (%d)\n",d->d_name.name,
                  atomic_read(&d->d_count));
@@ -478,9 +518,9 @@ void sysfs_remove_subdir(struct dentry * d)
 
 static void __sysfs_remove_dir(struct dentry *dentry)
 {
-        LIST_HEAD(removed);
-        struct sysfs_dirent * parent_sd;
-        struct sysfs_dirent * sd, * tmp;
+        struct sysfs_dirent *removed = NULL;
+        struct sysfs_dirent *parent_sd;
+        struct sysfs_dirent **pos;
 
         if (!dentry)
                 return;
@@ -488,15 +528,25 @@ static void __sysfs_remove_dir(struct dentry *dentry)
         pr_debug("sysfs %s: removing dir\n",dentry->d_name.name);
         mutex_lock(&dentry->d_inode->i_mutex);
         parent_sd = dentry->d_fsdata;
-        list_for_each_entry_safe(sd, tmp, &parent_sd->s_children, s_sibling) {
-                if (!sd->s_type || !(sd->s_type & SYSFS_NOT_PINNED))
-                        continue;
-                list_move(&sd->s_sibling, &removed);
+        pos = &parent_sd->s_children;
+        while (*pos) {
+                struct sysfs_dirent *sd = *pos;
+
+                if (sd->s_type && (sd->s_type & SYSFS_NOT_PINNED)) {
+                        *pos = sd->s_sibling;
+                        sd->s_sibling = removed;
+                        removed = sd;
+                } else
+                        pos = &(*pos)->s_sibling;
         }
         mutex_unlock(&dentry->d_inode->i_mutex);
 
-        list_for_each_entry_safe(sd, tmp, &removed, s_sibling) {
-                list_del_init(&sd->s_sibling);
+        while (removed) {
+                struct sysfs_dirent *sd = removed;
+
+                removed = sd->s_sibling;
+                sd->s_sibling = NULL;
+
                 sysfs_drop_dentry(sd);
                 sysfs_deactivate(sd);
                 sysfs_put(sd);
@@ -577,11 +627,11 @@ int sysfs_rename_dir(struct kobject * kobj, struct dentry *new_parent,
         d_add(new_dentry, NULL);
         d_move(kobj->dentry, new_dentry);
 
-        list_del_init(&sd->s_sibling);
+        sysfs_unlink_sibling(sd);
         sysfs_get(parent_sd);
         sysfs_put(sd->s_parent);
         sd->s_parent = parent_sd;
-        list_add(&sd->s_sibling, &parent_sd->s_children);
+        sysfs_link_sibling(sd);
 
         error = 0;
         goto out_unlock;
@@ -633,11 +683,11 @@ again:
         dput(new_dentry);
 
         /* Remove from old parent's list and insert into new parent's list. */
-        list_del_init(&sd->s_sibling);
+        sysfs_unlink_sibling(sd);
         sysfs_get(new_parent_sd);
         sysfs_put(sd->s_parent);
         sd->s_parent = new_parent_sd;
-        list_add(&sd->s_sibling, &new_parent_sd->s_children);
+        sysfs_link_sibling(sd);
 
 out:
         mutex_unlock(&new_parent_dentry->d_inode->i_mutex);
@@ -668,7 +718,7 @@ static int sysfs_dir_close(struct inode *inode, struct file *file)
         struct sysfs_dirent * cursor = file->private_data;
 
         mutex_lock(&dentry->d_inode->i_mutex);
-        list_del_init(&cursor->s_sibling);
+        sysfs_unlink_sibling(cursor);
         mutex_unlock(&dentry->d_inode->i_mutex);
 
         release_sysfs_dirent(cursor);
@@ -687,7 +737,7 @@ static int sysfs_readdir(struct file * filp, void * dirent, filldir_t filldir)
         struct dentry *dentry = filp->f_path.dentry;
         struct sysfs_dirent * parent_sd = dentry->d_fsdata;
         struct sysfs_dirent *cursor = filp->private_data;
-        struct list_head *p, *q = &cursor->s_sibling;
+        struct sysfs_dirent **pos;
         ino_t ino;
         int i = filp->f_pos;
 
@@ -710,16 +760,21 @@ static int sysfs_readdir(struct file * filp, void * dirent, filldir_t filldir)
                         i++;
                         /* fallthrough */
                 default:
+                        pos = &parent_sd->s_children;
+                        while (*pos != cursor)
+                                pos = &(*pos)->s_sibling;
+
+                        /* unlink cursor */
+                        *pos = cursor->s_sibling;
+
                         if (filp->f_pos == 2)
-                                list_move(q, &parent_sd->s_children);
+                                pos = &parent_sd->s_children;
 
-                        for (p=q->next; p!= &parent_sd->s_children; p=p->next) {
-                                struct sysfs_dirent *next;
+                        for ( ; *pos; pos = &(*pos)->s_sibling) {
+                                struct sysfs_dirent *next = *pos;
                                 const char * name;
                                 int len;
 
-                                next = list_entry(p, struct sysfs_dirent,
-                                                   s_sibling);
                                 if (!next->s_type)
                                         continue;
 
@@ -729,12 +784,14 @@ static int sysfs_readdir(struct file * filp, void * dirent, filldir_t filldir)
 
                                 if (filldir(dirent, name, len, filp->f_pos, ino,
                                                  dt_type(next)) < 0)
-                                        return 0;
+                                        break;
 
-                                list_move(q, p);
-                                p = q;
                                 filp->f_pos++;
                         }
+
+                        /* put cursor back in */
+                        cursor->s_sibling = *pos;
+                        *pos = cursor;
         }
         return 0;
 }
@@ -759,20 +816,21 @@ static loff_t sysfs_dir_lseek(struct file * file, loff_t offset, int origin)
                 if (file->f_pos >= 2) {
                         struct sysfs_dirent *sd = dentry->d_fsdata;
                         struct sysfs_dirent *cursor = file->private_data;
-                        struct list_head *p;
+                        struct sysfs_dirent **pos;
                         loff_t n = file->f_pos - 2;
 
-                        list_del(&cursor->s_sibling);
-                        p = sd->s_children.next;
-                        while (n && p != &sd->s_children) {
-                                struct sysfs_dirent *next;
-                                next = list_entry(p, struct sysfs_dirent,
-                                                   s_sibling);
+                        sysfs_unlink_sibling(cursor);
+
+                        pos = &sd->s_children;
+                        while (n && *pos) {
+                                struct sysfs_dirent *next = *pos;
                                 if (next->s_type)
                                         n--;
-                                p = p->next;
+                                pos = &(*pos)->s_sibling;
                         }
-                        list_add_tail(&cursor->s_sibling, p);
+
+                        cursor->s_sibling = *pos;
+                        *pos = cursor;
                 }
         }
         mutex_unlock(&dentry->d_inode->i_mutex);
diff --git a/fs/sysfs/inode.c b/fs/sysfs/inode.c
index 3eab9c46a71b..732fd7f371e0 100644
--- a/fs/sysfs/inode.c
+++ b/fs/sysfs/inode.c
@@ -284,8 +284,8 @@ void sysfs_drop_dentry(struct sysfs_dirent *sd)
 
 int sysfs_hash_and_remove(struct dentry * dir, const char * name)
 {
-        struct sysfs_dirent * sd;
-        struct sysfs_dirent * parent_sd;
+        struct sysfs_dirent **pos, *sd;
+        struct sysfs_dirent *parent_sd = dir->d_fsdata;
         int found = 0;
 
         if (!dir)
@@ -295,13 +295,15 @@ int sysfs_hash_and_remove(struct dentry * dir, const char * name)
                 /* no inode means this hasn't been made visible yet */
                 return -ENOENT;
 
-        parent_sd = dir->d_fsdata;
         mutex_lock_nested(&dir->d_inode->i_mutex, I_MUTEX_PARENT);
-        list_for_each_entry(sd, &parent_sd->s_children, s_sibling) {
+        for (pos = &parent_sd->s_children; *pos; pos = &(*pos)->s_sibling) {
+                sd = *pos;
+
                 if (!sd->s_type)
                         continue;
                 if (!strcmp(sd->s_name, name)) {
-                        list_del_init(&sd->s_sibling);
+                        *pos = sd->s_sibling;
+                        sd->s_sibling = NULL;
                         found = 1;
                         break;
                 }
diff --git a/fs/sysfs/mount.c b/fs/sysfs/mount.c
index d0e160307c5d..4be9593ea000 100644
--- a/fs/sysfs/mount.c
+++ b/fs/sysfs/mount.c
@@ -26,11 +26,8 @@ static const struct super_operations sysfs_ops = {
 
 static struct sysfs_dirent sysfs_root = {
         .s_count        = ATOMIC_INIT(1),
-        .s_sibling      = LIST_HEAD_INIT(sysfs_root.s_sibling),
-        .s_children     = LIST_HEAD_INIT(sysfs_root.s_children),
         .s_type         = SYSFS_ROOT,
         .s_mode         = S_IFDIR | S_IRWXU | S_IRUGO | S_IXUGO,
-        .s_iattr        = NULL,
         .s_ino          = 1,
 };
 
diff --git a/fs/sysfs/sysfs.h b/fs/sysfs/sysfs.h
index ae006b070bf0..6f8aaf3805d2 100644
--- a/fs/sysfs/sysfs.h
+++ b/fs/sysfs/sysfs.h
@@ -23,8 +23,8 @@ struct sysfs_dirent {
         atomic_t                s_count;
         atomic_t                s_active;
         struct sysfs_dirent     * s_parent;
-        struct list_head        s_sibling;
-        struct list_head        s_children;
+        struct sysfs_dirent     * s_sibling;
+        struct sysfs_dirent     * s_children;
         const char              * s_name;
 
         union {