ext4: avoid deadlock on sync-mounted FS w/o journal

Processes hang forever on a sync-mounted ext2 file system that
is mounted with the ext4 module (default in Fedora 16).

I can reproduce this reliably by mounting an ext2 partition with
"-o sync" and opening a new file an that partition with vim. vim
will hang in "D" state forever.  The same happens on ext4 without
a journal.

I am attaching a small patch here that solves this issue for me.
In the sync mounted case without a journal,
ext4_handle_dirty_metadata() may call sync_dirty_buffer(), which
can't be called with buffer lock held.

Also move mb_cache_entry_release inside lock to avoid race
fixed previously by 8a2bfdcb ext[34]: EA block reference count racing fix
Note too that ext2 fixed this same problem in 2006 with
b2f49033 [PATCH] fix deadlock in ext2

Signed-off-by: Martin.Wilck@ts.fujitsu.com
[sandeen@redhat.com: move mb_cache_entry_release before unlock, edit commit msg]
Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>

1 files changed, 4 insertions, 3 deletions

diff --git a/fs/ext4/xattr.c b/fs/ext4/xattr.c
index 1bff752d1b59..3369157df852 100644
--- a/fs/ext4/xattr.c
+++ b/fs/ext4/xattr.c
@@ -484,18 +484,19 @@ ext4_xattr_release_block(handle_t *handle, struct inode *inode,
                 ext4_free_blocks(handle, inode, bh, 0, 1,
                                  EXT4_FREE_BLOCKS_METADATA |
                                  EXT4_FREE_BLOCKS_FORGET);
+                unlock_buffer(bh);
         } else {
                 le32_add_cpu(&BHDR(bh)->h_refcount, -1);
+                if (ce)
+                        mb_cache_entry_release(ce);
+                unlock_buffer(bh);
                 error = ext4_handle_dirty_metadata(handle, inode, bh);
                 if (IS_SYNC(inode))
                         ext4_handle_sync(handle);
                 dquot_free_block(inode, 1);
                 ea_bdebug(bh, "refcount now=%d; releasing",
                           le32_to_cpu(BHDR(bh)->h_refcount));
-                if (ce)
-                        mb_cache_entry_release(ce);
         }
-        unlock_buffer(bh);
 out:
         ext4_std_error(inode->i_sb, error);
         return;