Unionfs: prevent false lockdep warnings in stacking

A stackable file system like unionfs often performs an operation on a lower
file system, by calling a vfs_* method, having been called possibly by the
very same method from the VFS.  Both calls to the vfs_* method grab a lock
in the same lock class, and hence lockdep complains.  This warning is a
false positive in instances where unionfs only calls the vfs_* method on
lower objects; there's a strict lock ordering here: upper objects first,
then lower objects.

We want to prevent these false positives so that lockdep will not shutdown
so it'd still be able to warn us about potentially true locking problems.
So, we temporarily turn off lockdep ONLY AROUND the calls to vfs methods to
which we pass lower objects, and only for those instances where lockdep
complained.  While this solution may seem unclean, it is not without
precedent: other places in the kernel also do similar temporary disabling,
of course after carefully having checked that it is the right thing to do.

In the long run, lockdep needs to be taught how to handle about stacking.
Then this patch can be removed.  It is likely that such lockdep-stacking
support will do essentially the same as this patch: consider the same
ordering (upper then lower) and consider upper vs. lower locks to be in
different classes.

Signed-off-by: Erez Zadok <ezk@cs.sunysb.edu>

diff --git a/Documentation/filesystems/unionfs/issues.txt b/Documentation/filesystems/unionfs/issues.txt
index bb6ab052c17c7694cdeec96b42d5018369d268ad..f4b7e7e26954a22bb27a74f4941d071facadf20e 100644 (file)
--- a/Documentation/filesystems/unionfs/issues.txt
+++ b/Documentation/filesystems/unionfs/issues.txt
@@ -17,8 +17,12 @@ KNOWN Unionfs 2.x ISSUES:
    an upper object, and then a lower object, in a strict order to avoid
    locking problems; in addition, Unionfs, as a fan-out file system, may
    have to lock several lower inodes.  We are currently looking into Lockdep
-   to see how to make it aware of stackable file systems.  In the meantime,
-   if you get any warnings from Lockdep, you can safely ignore them (or feel
-   free to report them to the Unionfs maintainers, just to be sure).
+   to see how to make it aware of stackable file systems.  For now, we
+   temporarily disable lockdep when calling vfs methods on lower objects,
+   but only for those places where lockdep complained.  While this solution
+   may seem unclean, it is not without precedent: other places in the kernel
+   also do similar temporary disabling, of course after carefully having
+   checked that it is the right thing to do.  Anyway, you get any warnings
+   from Lockdep, please report them to the Unionfs maintainers.
 
 For more information, see <http://unionfs.filesystems.org/>.

diff --git a/fs/unionfs/copyup.c b/fs/unionfs/copyup.c
index f48209f62c5ed685b9256919b2b20175fa944383..0012cafbc080c08db498e892bfab9b4095c937ae 100644 (file)
--- a/fs/unionfs/copyup.c
+++ b/fs/unionfs/copyup.c
@@ -297,11 +297,14 @@ static int __copyup_reg_data(struct dentry *dentry,
                        break;
                }
 
+               /* see Documentation/filesystems/unionfs/issues.txt */
+               lockdep_off();
                write_bytes =
                        output_file->f_op->write(output_file,
                                                 (char __user *)buf,
                                                 read_bytes,
                                                 &output_file->f_pos);
+               lockdep_on();
                if ((write_bytes < 0) || (write_bytes < read_bytes)) {
                        err = write_bytes;
                        break;

diff --git a/fs/unionfs/inode.c b/fs/unionfs/inode.c
index 4cda6da0dbc0738604b26c61b8d388b47d01ef27..b84680dbee1fe585b4b2788971c81d327ddebaa3 100644 (file)
--- a/fs/unionfs/inode.c
+++ b/fs/unionfs/inode.c
@@ -80,7 +80,10 @@ static int unionfs_create(struct inode *parent, struct dentry *dentry,
                        struct dentry *lower_dir_dentry;
 
                        lower_dir_dentry = lock_parent(wh_dentry);
+                       /* see Documentation/filesystems/unionfs/issues.txt */
+                       lockdep_off();
                        err = vfs_unlink(lower_dir_dentry->d_inode, wh_dentry);
+                       lockdep_on();
                        unlock_dir(lower_dir_dentry);
 
                        /*
@@ -262,9 +265,13 @@ static int unionfs_link(struct dentry *old_dentry, struct inode *dir,
                /* found a .wh.foo entry, unlink it and then call vfs_link() */
                lower_dir_dentry = lock_parent(whiteout_dentry);
                err = is_robranch_super(new_dentry->d_sb, dbstart(new_dentry));
-               if (!err)
+               if (!err) {
+                       /* see Documentation/filesystems/unionfs/issues.txt */
+                       lockdep_off();
                        err = vfs_unlink(lower_dir_dentry->d_inode,
                                         whiteout_dentry);
+                       lockdep_on();
+               }
 
                fsstack_copy_attr_times(dir, lower_dir_dentry->d_inode);
                dir->i_nlink = unionfs_get_nlinks(dir);
@@ -291,9 +298,13 @@ static int unionfs_link(struct dentry *old_dentry, struct inode *dir,
        BUG_ON(dbstart(old_dentry) != dbstart(new_dentry));
        lower_dir_dentry = lock_parent(lower_new_dentry);
        err = is_robranch(old_dentry);
-       if (!err)
+       if (!err) {
+               /* see Documentation/filesystems/unionfs/issues.txt */
+               lockdep_off();
                err = vfs_link(lower_old_dentry, lower_dir_dentry->d_inode,
                               lower_new_dentry);
+               lockdep_on();
+       }
        unlock_dir(lower_dir_dentry);
 
 docopyup:
@@ -316,10 +327,16 @@ docopyup:
                                        unionfs_lower_dentry(old_dentry);
                                lower_dir_dentry =
                                        lock_parent(lower_new_dentry);
+                               /*
+                                * see
+                                * Documentation/filesystems/unionfs/issues.txt
+                                */
+                               lockdep_off();
                                /* do vfs_link */
                                err = vfs_link(lower_old_dentry,
                                               lower_dir_dentry->d_inode,
                                               lower_new_dentry);
+                               lockdep_on();
                                unlock_dir(lower_dir_dentry);
                                goto check_link;
                        }

diff --git a/fs/unionfs/rename.c b/fs/unionfs/rename.c
index 452d1e7778298f9d52f0851e8ba5ae481df4b64b..8b04acf53174c09a0a6dd1060b3d4a13b1d39052 100644 (file)
--- a/fs/unionfs/rename.c
+++ b/fs/unionfs/rename.c
@@ -90,16 +90,14 @@ static int __unionfs_rename(struct inode *old_dir, struct dentry *old_dentry,
                dput(lower_wh_dentry);
        }
 
+       err = is_robranch_super(old_dentry->d_sb, bindex);
+       if (err)
+               goto out;
+
        dget(lower_old_dentry);
        lower_old_dir_dentry = dget_parent(lower_old_dentry);
        lower_new_dir_dentry = dget_parent(lower_new_dentry);
 
-       lock_rename(lower_old_dir_dentry, lower_new_dir_dentry);
-
-       err = is_robranch_super(old_dentry->d_sb, bindex);
-       if (err)
-               goto out_unlock;
-
        /*
         * ready to whiteout for old_dentry. caller will create the actual
         * whiteout, and must dput(*wh_old)
@@ -110,7 +108,7 @@ static int __unionfs_rename(struct inode *old_dir, struct dentry *old_dentry,
                                      old_dentry->d_name.len);
                err = PTR_ERR(whname);
                if (unlikely(IS_ERR(whname)))
-                       goto out_unlock;
+                       goto out_dput;
                *wh_old = lookup_one_len(whname, lower_old_dir_dentry,
                                         old_dentry->d_name.len +
                                         UNIONFS_WHLEN);
@@ -118,16 +116,19 @@ static int __unionfs_rename(struct inode *old_dir, struct dentry *old_dentry,
                err = PTR_ERR(*wh_old);
                if (IS_ERR(*wh_old)) {
                        *wh_old = NULL;
-                       goto out_unlock;
+                       goto out_dput;
                }
        }
 
+       /* see Documentation/filesystems/unionfs/issues.txt */
+       lockdep_off();
+       lock_rename(lower_old_dir_dentry, lower_new_dir_dentry);
        err = vfs_rename(lower_old_dir_dentry->d_inode, lower_old_dentry,
                         lower_new_dir_dentry->d_inode, lower_new_dentry);
-
-out_unlock:
        unlock_rename(lower_old_dir_dentry, lower_new_dir_dentry);
+       lockdep_on();
 
+out_dput:
        dput(lower_old_dir_dentry);
        dput(lower_new_dir_dentry);
        dput(lower_old_dentry);

diff --git a/fs/unionfs/super.c b/fs/unionfs/super.c
index 204f87406a9d0704db2a45e680baff235f15de96..fcea65dc0f23582f3170d6604c2f8cd401c975b1 100644 (file)
--- a/fs/unionfs/super.c
+++ b/fs/unionfs/super.c
@@ -839,7 +839,11 @@ static void unionfs_clear_inode(struct inode *inode)
                        lower_inode = unionfs_lower_inode_idx(inode, bindex);
                        if (!lower_inode)
                                continue;
+                       unionfs_set_lower_inode_idx(inode, bindex, NULL);
+                       /* see Documentation/filesystems/unionfs/issues.txt */
+                       lockdep_off();
                        iput(lower_inode);
+                       lockdep_on();
                }
        }
 

diff --git a/fs/unionfs/unlink.c b/fs/unionfs/unlink.c
index 423ff36185588ec67dc65082bb083a270d8b7e8b..677a5aee5bc4fd6f37cd638b06bdbb7428ae3464 100644 (file)
--- a/fs/unionfs/unlink.c
+++ b/fs/unionfs/unlink.c
@@ -41,8 +41,12 @@ static int unionfs_unlink_whiteout(struct inode *dir, struct dentry *dentry)
        /* avoid destroying the lower inode if the file is in use */
        dget(lower_dentry);
        err = is_robranch_super(dentry->d_sb, bindex);
-       if (!err)
+       if (!err) {
+               /* see Documentation/filesystems/unionfs/issues.txt */
+               lockdep_off();
                err = vfs_unlink(lower_dir_dentry->d_inode, lower_dentry);
+               lockdep_on();
+       }
        /* if vfs_unlink succeeded, update our inode's times */
        if (!err)
                unionfs_copy_attr_times(dentry->d_inode);
@@ -139,8 +143,12 @@ static int unionfs_rmdir_first(struct inode *dir, struct dentry *dentry,
        /* avoid destroying the lower inode if the file is in use */
        dget(lower_dentry);
        err = is_robranch(dentry);
-       if (!err)
+       if (!err) {
+               /* see Documentation/filesystems/unionfs/issues.txt */
+               lockdep_off();
                err = vfs_rmdir(lower_dir_dentry->d_inode, lower_dentry);
+               lockdep_on();
+       }
        dput(lower_dentry);
 
        fsstack_copy_attr_times(dir, lower_dir_dentry->d_inode);