libfs.c: new helper - next_positive()

Return nth positive child after given or NULL if there's
less than n left.  dcache_readdir() and dcache_dir_lseek()
switched to it.

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

diff --git a/fs/libfs.c b/fs/libfs.c
index f56acb1..b05b74a 100644 (file)
--- a/fs/libfs.c
+++ b/fs/libfs.c
@@ -84,6 +84,39 @@ int dcache_dir_close(struct inode *inode, struct file *file)
 }
 EXPORT_SYMBOL(dcache_dir_close);
 
+/* parent is locked at least shared */
+static struct dentry *next_positive(struct dentry *parent,
+                                   struct list_head *from,
+                                   int count)
+{
+       struct dentry *res = NULL;
+       struct list_head *p;
+
+       spin_lock(&parent->d_lock);
+       for (p = from->next; p != &parent->d_subdirs; p = p->next) {
+               struct dentry *d = list_entry(p, struct dentry, d_child);
+               if (simple_positive(d) && !--count) {
+                       res = d;
+                       break;
+               }
+       }
+       spin_unlock(&parent->d_lock);
+       return res;
+}
+
+static void move_cursor(struct dentry *cursor, struct list_head *after)
+{
+       struct dentry *parent = cursor->d_parent;
+
+       spin_lock(&parent->d_lock);
+       __list_del(cursor->d_child.prev, cursor->d_child.next);
+       if (after)
+               list_add(&cursor->d_child, after);
+       else
+               list_add_tail(&cursor->d_child, &parent->d_subdirs);
+       spin_unlock(&parent->d_lock);
+}
+
 loff_t dcache_dir_lseek(struct file *file, loff_t offset, int whence)
 {
        struct dentry *dentry = file->f_path.dentry;
@@ -99,24 +132,13 @@ loff_t dcache_dir_lseek(struct file *file, loff_t offset, int whence)
        if (offset != file->f_pos) {
                file->f_pos = offset;
                if (file->f_pos >= 2) {
-                       struct list_head *p;
                        struct dentry *cursor = file->private_data;
+                       struct dentry *to;
                        loff_t n = file->f_pos - 2;
 
                        inode_lock_shared(dentry->d_inode);
-                       spin_lock(&dentry->d_lock);
-                       /* d_lock not required for cursor */
-                       list_del(&cursor->d_child);
-                       p = dentry->d_subdirs.next;
-                       while (n && p != &dentry->d_subdirs) {
-                               struct dentry *next;
-                               next = list_entry(p, struct dentry, d_child);
-                               if (simple_positive(next))
-                                       n--;
-                               p = p->next;
-                       }
-                       list_add_tail(&cursor->d_child, p);
-                       spin_unlock(&dentry->d_lock);
+                       to = next_positive(dentry, &dentry->d_subdirs, n);
+                       move_cursor(cursor, to ? &to->d_child : NULL);
                        inode_unlock_shared(dentry->d_inode);
                }
        }
@@ -140,30 +162,25 @@ int dcache_readdir(struct file *file, struct dir_context *ctx)
 {
        struct dentry *dentry = file->f_path.dentry;
        struct dentry *cursor = file->private_data;
-       struct list_head *p, *q = &cursor->d_child;
+       struct list_head *p = &cursor->d_child;
+       struct dentry *next;
+       bool moved = false;
 
        if (!dir_emit_dots(file, ctx))
                return 0;
-       spin_lock(&dentry->d_lock);
-       if (ctx->pos == 2)
-               list_move(q, &dentry->d_subdirs);
 
-       for (p = q->next; p != &dentry->d_subdirs; p = p->next) {
-               struct dentry *next = list_entry(p, struct dentry, d_child);
-               if (!simple_positive(next))
-                       continue;
-
-               spin_unlock(&dentry->d_lock);
+       if (ctx->pos == 2)
+               p = &dentry->d_subdirs;
+       while ((next = next_positive(dentry, p, 1)) != NULL) {
                if (!dir_emit(ctx, next->d_name.name, next->d_name.len,
                              d_inode(next)->i_ino, dt_type(d_inode(next))))
-                       return 0;
-               spin_lock(&dentry->d_lock);
-               /* next is still alive */
-               list_move(q, p);
-               p = q;
+                       break;
+               moved = true;
+               p = &next->d_child;
                ctx->pos++;
        }
-       spin_unlock(&dentry->d_lock);
+       if (moved)
+               move_cursor(cursor, p);
        return 0;
 }
 EXPORT_SYMBOL(dcache_readdir);