configfs: Introduce configfs_dirent_lock

This patch introduces configfs_dirent_lock spinlock to protect configfs_dirent
traversals against linkage mutations (add/del/move). This will allow
configfs_detach_prep() to avoid locking i_mutexes.

Locking rules for configfs_dirent linkage mutations are the same plus the
requirement of taking configfs_dirent_lock. For configfs_dirent walking, one can
either take appropriate i_mutex as before, or take configfs_dirent_lock.

The spinlock could actually be a mutex, but the critical sections are either
O(1) or should not be too long (default groups walking in last patch).

ChangeLog:
  - Clarify the comment on configfs_dirent_lock usage
  - Move sd->s_element init before linking the new dirent
  - In lseek(), do not release configfs_dirent_lock before the dirent is
    relinked.

Signed-off-by: Louis Rilling <Louis.Rilling@kerlabs.com>
Signed-off-by: Joel Becker <joel.becker@oracle.com>

4 files changed, 34 insertions, 1 deletions

diff --git a/fs/configfs/configfs_internal.h b/fs/configfs/configfs_internal.h
index cca98609aa7f..5a33b58e66da 100644
--- a/fs/configfs/configfs_internal.h
+++ b/fs/configfs/configfs_internal.h
@@ -26,6 +26,7 @@
 
 #include <linux/slab.h>
 #include <linux/list.h>
+#include <linux/spinlock.h>
 
 struct configfs_dirent {
         atomic_t                s_count;
@@ -49,6 +50,8 @@ struct configfs_dirent {
 #define CONFIGFS_USET_DROPPING  0x0100
 #define CONFIGFS_NOT_PINNED     (CONFIGFS_ITEM_ATTR)
 
+extern spinlock_t configfs_dirent_lock;
+
 extern struct vfsmount * configfs_mount;
 extern struct kmem_cache *configfs_dir_cachep;
 
diff --git a/fs/configfs/dir.c b/fs/configfs/dir.c
index a48dc7dd8765..2619f485bc3d 100644
--- a/fs/configfs/dir.c
+++ b/fs/configfs/dir.c
@@ -35,6 +35,14 @@
 #include "configfs_internal.h"
 
 DECLARE_RWSEM(configfs_rename_sem);
+/*
+ * Protects mutations of configfs_dirent linkage together with proper i_mutex
+ * Mutators of configfs_dirent linkage must *both* have the proper inode locked
+ * and configfs_dirent_lock locked, in that order.
+ * This allows one to safely traverse configfs_dirent trees without having to
+ * lock inodes.
+ */
+DEFINE_SPINLOCK(configfs_dirent_lock);
 
 static void configfs_d_iput(struct dentry * dentry,
                             struct inode * inode)
@@ -79,8 +87,10 @@ static struct configfs_dirent *configfs_new_dirent(struct configfs_dirent * pare
         atomic_set(&sd->s_count, 1);
         INIT_LIST_HEAD(&sd->s_links);
         INIT_LIST_HEAD(&sd->s_children);
-        list_add(&sd->s_sibling, &parent_sd->s_children);
         sd->s_element = element;
+        spin_lock(&configfs_dirent_lock);
+        list_add(&sd->s_sibling, &parent_sd->s_children);
+        spin_unlock(&configfs_dirent_lock);
 
         return sd;
 }
@@ -173,7 +183,9 @@ static int create_dir(struct config_item * k, struct dentry * p,
                 } else {
                         struct configfs_dirent *sd = d->d_fsdata;
                         if (sd) {
+                                spin_lock(&configfs_dirent_lock);
                                 list_del_init(&sd->s_sibling);
+                                spin_unlock(&configfs_dirent_lock);
                                 configfs_put(sd);
                         }
                 }
@@ -224,7 +236,9 @@ int configfs_create_link(struct configfs_symlink *sl,
                 else {
                         struct configfs_dirent *sd = dentry->d_fsdata;
                         if (sd) {
+                                spin_lock(&configfs_dirent_lock);
                                 list_del_init(&sd->s_sibling);
+                                spin_unlock(&configfs_dirent_lock);
                                 configfs_put(sd);
                         }
                 }
@@ -238,7 +252,9 @@ static void remove_dir(struct dentry * d)
         struct configfs_dirent * sd;
 
         sd = d->d_fsdata;
+        spin_lock(&configfs_dirent_lock);
         list_del_init(&sd->s_sibling);
+        spin_unlock(&configfs_dirent_lock);
         configfs_put(sd);
         if (d->d_inode)
                 simple_rmdir(parent->d_inode,d);
@@ -410,7 +426,9 @@ static void detach_attrs(struct config_item * item)
         list_for_each_entry_safe(sd, tmp, &parent_sd->s_children, s_sibling) {
                 if (!sd->s_element || !(sd->s_type & CONFIGFS_NOT_PINNED))
                         continue;
+                spin_lock(&configfs_dirent_lock);
                 list_del_init(&sd->s_sibling);
+                spin_unlock(&configfs_dirent_lock);
                 configfs_drop_dentry(sd, dentry);
                 configfs_put(sd);
         }
@@ -1268,7 +1286,9 @@ static int configfs_dir_close(struct inode *inode, struct file *file)
         struct configfs_dirent * cursor = file->private_data;
 
         mutex_lock(&dentry->d_inode->i_mutex);
+        spin_lock(&configfs_dirent_lock);
         list_del_init(&cursor->s_sibling);
+        spin_unlock(&configfs_dirent_lock);
         mutex_unlock(&dentry->d_inode->i_mutex);
 
         release_configfs_dirent(cursor);
@@ -1308,7 +1328,9 @@ static int configfs_readdir(struct file * filp, void * dirent, filldir_t filldir
                         /* fallthrough */
                 default:
                         if (filp->f_pos == 2) {
+                                spin_lock(&configfs_dirent_lock);
                                 list_move(q, &parent_sd->s_children);
+                                spin_unlock(&configfs_dirent_lock);
                         }
                         for (p=q->next; p!= &parent_sd->s_children; p=p->next) {
                                 struct configfs_dirent *next;
@@ -1331,7 +1353,9 @@ static int configfs_readdir(struct file * filp, void * dirent, filldir_t filldir
                                                  dt_type(next)) < 0)
                                         return 0;
 
+                                spin_lock(&configfs_dirent_lock);
                                 list_move(q, p);
+                                spin_unlock(&configfs_dirent_lock);
                                 p = q;
                                 filp->f_pos++;
                         }
@@ -1362,6 +1386,7 @@ static loff_t configfs_dir_lseek(struct file * file, loff_t offset, int origin)
                         struct list_head *p;
                         loff_t n = file->f_pos - 2;
 
+                        spin_lock(&configfs_dirent_lock);
                         list_del(&cursor->s_sibling);
                         p = sd->s_children.next;
                         while (n && p != &sd->s_children) {
@@ -1373,6 +1398,7 @@ static loff_t configfs_dir_lseek(struct file * file, loff_t offset, int origin)
                                 p = p->next;
                         }
                         list_add_tail(&cursor->s_sibling, p);
+                        spin_unlock(&configfs_dirent_lock);
                 }
         }
         mutex_unlock(&dentry->d_inode->i_mutex);
diff --git a/fs/configfs/inode.c b/fs/configfs/inode.c
index b9a1d810346d..4803ccc94480 100644
--- a/fs/configfs/inode.c
+++ b/fs/configfs/inode.c
@@ -247,7 +247,9 @@ void configfs_hash_and_remove(struct dentry * dir, const char * name)
                 if (!sd->s_element)
                         continue;
                 if (!strcmp(configfs_get_name(sd), name)) {
+                        spin_lock(&configfs_dirent_lock);
                         list_del_init(&sd->s_sibling);
+                        spin_unlock(&configfs_dirent_lock);
                         configfs_drop_dentry(sd, dir);
                         configfs_put(sd);
                         break;
diff --git a/fs/configfs/symlink.c b/fs/configfs/symlink.c
index 2a731ef5f305..676c84c416da 100644
--- a/fs/configfs/symlink.c
+++ b/fs/configfs/symlink.c
@@ -169,7 +169,9 @@ int configfs_unlink(struct inode *dir, struct dentry *dentry)
         parent_item = configfs_get_config_item(dentry->d_parent);
         type = parent_item->ci_type;
 
+        spin_lock(&configfs_dirent_lock);
         list_del_init(&sd->s_sibling);
+        spin_unlock(&configfs_dirent_lock);
         configfs_drop_dentry(sd, dentry->d_parent);
         dput(dentry);
         configfs_put(sd);