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 3df9b190d042e8a374a322d280cbb8b52653c722..4890f423041ca6ff3140d2286dfc2ecd7c7090f5 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 8b70acab741eb54476d6874bed8d692b3afff394..45bcf89663cd0122007744abec7d6c59d90ccdb1 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);