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

commit c1bb05a upstream.

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 8a2bfdc ext[34]: EA block reference count racing fix
Note too that ext2 fixed this same problem in 2006 with
b2f4903 [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>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
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1 parent 167a899 commit 339972629a2fbfe69d65993ba2e4071d5dd7d7c3 Eric Sandeen committed with koenkooi Feb 21, 2012
Showing with 4 additions and 3 deletions.
  1. +4 −3 fs/ext4/xattr.c
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@@ -487,18 +487,19 @@ 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;

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