GIT: unionfs2-2.6.27.y: VFS/fs_stack: use locking around i_size_write in 32-bit systems

[Erez Zadok]

commit 46a69f0b644d9c9213b4febbb49bf06f06979301
Author: Hugh Dickins <hugh at veritas.com>
Date:   Tue Dec 25 16:39:35 2007 -0500

    VFS/fs_stack: use locking around i_size_write in 32-bit systems
    
    LTP's iogen01 doio tests hang nicely on 32-bit SMP when /tmp is a unionfs
    mount of a tmpfs.  See the comment on i_size_write in linux/fs.h: it needs
    to be locked, otherwise i_size_read can spin forever waiting for a lost
    seqcount update.
    
    Most filesystems are already holding i_mutex for this, but unionfs calls
    fsstack_copy_inode_size from many places, not necessarily holding i_mutex.
    Use the low-level i_lock within fsstack_copy_inode_size when 32-bit SMP.
    
    Checked the entire unionfs code to ensure this is the right fix for
    i_size_write().
    
    Also compared to what other file systems do when they have to handle inodes,
    esp. not their own inodes (e.g., network file systems have to access the
    exported file system's inodes).  Found out that most such file systems not just
    don't lock around i_size_write, but they don't even use i_size_read or
    i_size_write to access the inode's size.
    
    CC: Mike Halcrow <mhalcrow at us.ibm.com>
    
    Signed-off-by: Hugh Dickins <hugh at veritas.com>
    Signed-off-by: Erez Zadok <ezk at cs.sunysb.edu>

diff --git a/fs/stack.c b/fs/stack.c
index 7913fe5..4336f2b 100644
--- a/fs/stack.c
+++ b/fs/stack.c
@@ -21,8 +21,14 @@
  */
 void fsstack_copy_inode_size(struct inode *dst, const struct inode *src)
 {
+#if BITS_PER_LONG == 32 && defined(CONFIG_SMP)
+	spin_lock(&dst->i_lock);
+#endif
 	i_size_write(dst, i_size_read(src));
 	dst->i_blocks = src->i_blocks;
+#if BITS_PER_LONG == 32 && defined(CONFIG_SMP)
+	spin_unlock(&dst->i_lock);
+#endif
 }
 EXPORT_SYMBOL_GPL(fsstack_copy_inode_size);