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 199e2b6629dfb023dfbed737dc55f3a62545d53b..f7a7f32f975b5c6d05dd78d4bac666ea6026d3bc 100644 (file)
--- a/fs/unionfs/copyup.c
+++ b/fs/unionfs/copyup.c
@@ -291,11 +291,14 @@ 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 ff8294a1a573393e051188242ea9a50aeddcc547..6b388cc9316124f3c8432204206b2c79f5639300 100644 (file)
--- a/fs/unionfs/inode.c
+++ b/fs/unionfs/inode.c
@@ -79,7 +79,10 @@ 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);
 
                        /*
@@ -253,9 +256,13 @@ 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);
@@ -282,9 +289,13 @@ 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:
@@ -307,10 +318,16 @@ 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 ac13feaf644176a231b011e5c617cd18a6b1f084..37b83f69e1b4f4245b02ba9f728e6f1f110bea11 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 631f7326365090e6a83e5b794361331329b20562..64a761158084efdf357ce3a91f1d36bdd5039ab7 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);