Unexport do_add_mount() and add in follow_automount(), not ->d_automount()

Unexport do_add_mount() and make ->d_automount() return the vfsmount to be
added rather than calling do_add_mount() itself.  follow_automount() will then
do the addition.

This slightly complicates things as ->d_automount() normally wants to add the
new vfsmount to an expiration list and start an expiration timer.  The problem
with that is that the vfsmount will be deleted if it has a refcount of 1 and
the timer will not repeat if the expiration list is empty.

To this end, we require the vfsmount to be returned from d_automount() with a
refcount of (at least) 2.  One of these refs will be dropped unconditionally.
In addition, follow_automount() must get a 3rd ref around the call to
do_add_mount() lest it eat a ref and return an error, leaving the mount we
have open to being expired as we would otherwise have only 1 ref on it.

d_automount() should also add the the vfsmount to the expiration list (by
calling mnt_set_expiry()) and start the expiration timer before returning, if
this mechanism is to be used.  The vfsmount will be unlinked from the
expiration list by follow_automount() if do_add_mount() fails.

This patch also fixes the call to do_add_mount() for AFS to propagate the mount
flags from the parent vfsmount.

Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

8 files changed, 101 insertions, 89 deletions

diff --git a/Documentation/filesystems/vfs.txt b/Documentation/filesystems/vfs.txt
index 3c4b2f1b64d0..94cf97b901d7 100644
--- a/Documentation/filesystems/vfs.txt
+++ b/Documentation/filesystems/vfs.txt
@@ -933,15 +933,20 @@ struct dentry_operations {
         dynamic_dname() helper function is provided to take care of this.
 
   d_automount: called when an automount dentry is to be traversed (optional).
-        This should create a new VFS mount record, mount it on the directory
-        and return the record to the caller.  The caller is supplied with a
-        path parameter giving the automount directory to describe the automount
-        target and the parent VFS mount record to provide inheritable mount
-        parameters.  NULL should be returned if someone else managed to make
-        the automount first.  If the automount failed, then an error code
-        should be returned.  If -EISDIR is returned, then the directory will
-        be treated as an ordinary directory and returned to pathwalk to
-        continue walking.
+        This should create a new VFS mount record and return the record to the
+        caller.  The caller is supplied with a path parameter giving the
+        automount directory to describe the automount target and the parent
+        VFS mount record to provide inheritable mount parameters.  NULL should
+        be returned if someone else managed to make the automount first.  If
+        the vfsmount creation failed, then an error code should be returned.
+        If -EISDIR is returned, then the directory will be treated as an
+        ordinary directory and returned to pathwalk to continue walking.
+
+        If a vfsmount is returned, the caller will attempt to mount it on the
+        mountpoint and will remove the vfsmount from its expiration list in
+        the case of failure.  The vfsmount should be returned with 2 refs on
+        it to prevent automatic expiration - the caller will clean up the
+        additional ref.
 
         This function is only used if DCACHE_NEED_AUTOMOUNT is set on the
         dentry.  This is set by __d_instantiate() if S_AUTOMOUNT is set on the
diff --git a/fs/afs/mntpt.c b/fs/afs/mntpt.c
index d23b2e344a78..aa59184151d0 100644
--- a/fs/afs/mntpt.c
+++ b/fs/afs/mntpt.c
@@ -241,7 +241,6 @@ error_no_devname:
 struct vfsmount *afs_d_automount(struct path *path)
 {
         struct vfsmount *newmnt;
-        int err;
 
         _enter("{%s,%s}", path->mnt->mnt_devname, path->dentry->d_name.name);
 
@@ -249,24 +248,12 @@ struct vfsmount *afs_d_automount(struct path *path)
         if (IS_ERR(newmnt))
                 return newmnt;
 
-        mntget(newmnt);
-        err = do_add_mount(newmnt, path, MNT_SHRINKABLE, &afs_vfsmounts);
-        switch (err) {
-        case 0:
-                queue_delayed_work(afs_wq, &afs_mntpt_expiry_timer,
-                                   afs_mntpt_expiry_timeout * HZ);
-                _leave(" = %p {%s}", newmnt, newmnt->mnt_devname);
-                return newmnt;
-        case -EBUSY:
-                /* someone else made a mount here whilst we were busy */
-                mntput(newmnt);
-                _leave(" = NULL [EBUSY]");
-                return NULL;
-        default:
-                mntput(newmnt);
-                _leave(" = %d", err);
-                return ERR_PTR(err);
-        }
+        mntget(newmnt); /* prevent immediate expiration */
+        mnt_set_expiry(newmnt, &afs_vfsmounts);
+        queue_delayed_work(afs_wq, &afs_mntpt_expiry_timer,
+                           afs_mntpt_expiry_timeout * HZ);
+        _leave(" = %p {%s}", newmnt, newmnt->mnt_devname);
+        return newmnt;
 }
 
 /*
diff --git a/fs/cifs/cifs_dfs_ref.c b/fs/cifs/cifs_dfs_ref.c
index 0fc163808de3..7ed36536e754 100644
--- a/fs/cifs/cifs_dfs_ref.c
+++ b/fs/cifs/cifs_dfs_ref.c
@@ -351,7 +351,6 @@ free_xid:
 struct vfsmount *cifs_dfs_d_automount(struct path *path)
 {
         struct vfsmount *newmnt;
-        int err;
 
         cFYI(1, "in %s", __func__);
 
@@ -361,25 +360,12 @@ struct vfsmount *cifs_dfs_d_automount(struct path *path)
                 return newmnt;
         }
 
-        mntget(newmnt);
-        err = do_add_mount(newmnt, path, path->mnt->mnt_flags | MNT_SHRINKABLE,
-                           &cifs_dfs_automount_list);
-        switch (err) {
-        case 0:
-                schedule_delayed_work(&cifs_dfs_automount_task,
-                                      cifs_dfs_mountpoint_expiry_timeout);
-                cFYI(1, "leaving %s [ok]" , __func__);
-                return newmnt;
-        case -EBUSY:
-                /* someone else made a mount here whilst we were busy */
-                mntput(newmnt);
-                cFYI(1, "leaving %s [EBUSY]" , __func__);
-                return NULL;
-        default:
-                mntput(newmnt);
-                cFYI(1, "leaving %s [error %d]" , __func__, err);
-                return ERR_PTR(err);
-        }
+        mntget(newmnt); /* prevent immediate expiration */
+        mnt_set_expiry(newmnt, &cifs_dfs_automount_list);
+        schedule_delayed_work(&cifs_dfs_automount_task,
+                              cifs_dfs_mountpoint_expiry_timeout);
+        cFYI(1, "leaving %s [ok]" , __func__);
+        return newmnt;
 }
 
 const struct inode_operations cifs_dfs_referral_inode_operations = {
diff --git a/fs/internal.h b/fs/internal.h
index 9687c2ee2735..4931060fd089 100644
--- a/fs/internal.h
+++ b/fs/internal.h
@@ -70,6 +70,8 @@ extern void mnt_set_mountpoint(struct vfsmount *, struct dentry *,
 extern void release_mounts(struct list_head *);
 extern void umount_tree(struct vfsmount *, int, struct list_head *);
 extern struct vfsmount *copy_tree(struct vfsmount *, struct dentry *, int);
+extern int do_add_mount(struct vfsmount *, struct path *, int);
+extern void mnt_clear_expiry(struct vfsmount *);
 
 extern void __init mnt_init(void);
 
diff --git a/fs/namei.c b/fs/namei.c
index 5c89695ae1e4..c2e37727e3ab 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -900,6 +900,7 @@ static int follow_automount(struct path *path, unsigned flags,
                             bool *need_mntput)
 {
         struct vfsmount *mnt;
+        int err;
 
         if (!path->dentry->d_op || !path->dentry->d_op->d_automount)
                 return -EREMOTE;
@@ -942,22 +943,49 @@ static int follow_automount(struct path *path, unsigned flags,
                         return -EREMOTE;
                 return PTR_ERR(mnt);
         }
+
         if (!mnt) /* mount collision */
                 return 0;
 
+        /* The new mount record should have at least 2 refs to prevent it being
+         * expired before we get a chance to add it
+         */
+        BUG_ON(mnt_get_count(mnt) < 2);
+
         if (mnt->mnt_sb == path->mnt->mnt_sb &&
             mnt->mnt_root == path->dentry) {
+                mnt_clear_expiry(mnt);
+                mntput(mnt);
                 mntput(mnt);
                 return -ELOOP;
         }
 
-        dput(path->dentry);
-        if (*need_mntput)
-                mntput(path->mnt);
-        path->mnt = mnt;
-        path->dentry = dget(mnt->mnt_root);
-        *need_mntput = true;
-        return 0;
+        /* We need to add the mountpoint to the parent.  The filesystem may
+         * have placed it on an expiry list, and so we need to make sure it
+         * won't be expired under us if do_add_mount() fails (do_add_mount()
+         * will eat a reference unconditionally).
+         */
+        mntget(mnt);
+        err = do_add_mount(mnt, path, path->mnt->mnt_flags | MNT_SHRINKABLE);
+        switch (err) {
+        case -EBUSY:
+                /* Someone else made a mount here whilst we were busy */
+                err = 0;
+        default:
+                mnt_clear_expiry(mnt);
+                mntput(mnt);
+                mntput(mnt);
+                return err;
+        case 0:
+                mntput(mnt);
+                dput(path->dentry);
+                if (*need_mntput)
+                        mntput(path->mnt);
+                path->mnt = mnt;
+                path->dentry = dget(mnt->mnt_root);
+                *need_mntput = true;
+                return 0;
+        }
 }
 
 /*
diff --git a/fs/namespace.c b/fs/namespace.c
index d94ccd6ddafd..bfcb701f9490 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -1925,15 +1925,14 @@ static int do_new_mount(struct path *path, char *type, int flags,
         if (IS_ERR(mnt))
                 return PTR_ERR(mnt);
 
-        return do_add_mount(mnt, path, mnt_flags, NULL);
+        return do_add_mount(mnt, path, mnt_flags);
 }
 
 /*
  * add a mount into a namespace's mount tree
- * - provide the option of adding the new mount to an expiration list
+ * - this unconditionally eats one of the caller's references to newmnt.
  */
-int do_add_mount(struct vfsmount *newmnt, struct path *path,
-                 int mnt_flags, struct list_head *fslist)
+int do_add_mount(struct vfsmount *newmnt, struct path *path, int mnt_flags)
 {
         int err;
 
@@ -1963,9 +1962,6 @@ int do_add_mount(struct vfsmount *newmnt, struct path *path,
         if ((err = graft_tree(newmnt, path)))
                 goto unlock;
 
-        if (fslist) /* add to the specified expiration list */
-                list_add_tail(&newmnt->mnt_expire, fslist);
-
         up_write(&namespace_sem);
         return 0;
 
@@ -1975,7 +1971,36 @@ unlock:
         return err;
 }
 
-EXPORT_SYMBOL_GPL(do_add_mount);
+/**
+ * mnt_set_expiry - Put a mount on an expiration list
+ * @mnt: The mount to list.
+ * @expiry_list: The list to add the mount to.
+ */
+void mnt_set_expiry(struct vfsmount *mnt, struct list_head *expiry_list)
+{
+        down_write(&namespace_sem);
+        br_write_lock(vfsmount_lock);
+
+        list_add_tail(&mnt->mnt_expire, expiry_list);
+
+        br_write_unlock(vfsmount_lock);
+        up_write(&namespace_sem);
+}
+EXPORT_SYMBOL(mnt_set_expiry);
+
+/*
+ * Remove a vfsmount from any expiration list it may be on
+ */
+void mnt_clear_expiry(struct vfsmount *mnt)
+{
+        if (!list_empty(&mnt->mnt_expire)) {
+                down_write(&namespace_sem);
+                br_write_lock(vfsmount_lock);
+                list_del_init(&mnt->mnt_expire);
+                br_write_unlock(vfsmount_lock);
+                up_write(&namespace_sem);
+        }
+}
 
 /*
  * process a list of expirable mountpoints with the intent of discarding any
diff --git a/fs/nfs/namespace.c b/fs/nfs/namespace.c
index f3fbb1bf3f18..f32b8603dca8 100644
--- a/fs/nfs/namespace.c
+++ b/fs/nfs/namespace.c
@@ -149,26 +149,10 @@ struct vfsmount *nfs_d_automount(struct path *path)
         if (IS_ERR(mnt))
                 goto out;
 
-        mntget(mnt);
-        err = do_add_mount(mnt, path, path->mnt->mnt_flags | MNT_SHRINKABLE,
-                           &nfs_automount_list);
-        switch (err) {
-        case 0:
-                dprintk("%s: done, success\n", __func__);
-                schedule_delayed_work(&nfs_automount_task, nfs_mountpoint_expiry_timeout);
-                break;
-        case -EBUSY:
-                /* someone else made a mount here whilst we were busy */
-                mntput(mnt);
-                dprintk("%s: done, collision\n", __func__);
-                mnt = NULL;
-                break;
-        default:
-                mntput(mnt);
-                dprintk("%s: done, error %d\n", __func__, err);
-                mnt = ERR_PTR(err);
-                break;
-        }
+        dprintk("%s: done, success\n", __func__);
+        mntget(mnt); /* prevent immediate expiration */
+        mnt_set_expiry(mnt, &nfs_automount_list);
+        schedule_delayed_work(&nfs_automount_task, nfs_mountpoint_expiry_timeout);
 
 out:
         nfs_free_fattr(fattr);
diff --git a/include/linux/mount.h b/include/linux/mount.h
index 1869ea24a739..af4765ea8fde 100644
--- a/include/linux/mount.h
+++ b/include/linux/mount.h
@@ -110,12 +110,7 @@ extern struct vfsmount *vfs_kern_mount(struct file_system_type *type,
                                       int flags, const char *name,
                                       void *data);
 
-struct nameidata;
-
-struct path;
-extern int do_add_mount(struct vfsmount *newmnt, struct path *path,
-                        int mnt_flags, struct list_head *fslist);
-
+extern void mnt_set_expiry(struct vfsmount *mnt, struct list_head *expiry_list);
 extern void mark_mounts_for_expiry(struct list_head *mounts);
 
 extern dev_t name_to_dev_t(char *name);