Buffer head async write

["Gorka Guardiola" <paurealkml@gmail.com>]

I am trying to  mark a buffer for async write, but I am  having a race condition
to unlock the buffer. I am doing.

       bh = getbmapst(bdev, nsect); //this function calls bread
       if (!bh) {
               printk(KERN_ALERT "I/O error, bitmap cannot be trusted\n");
               return -1;
       }

       lock_buffer(bh);

       byte = bh->b_data + byteoffset;
       *byte |= 1 << bitoffset;
       lock_page(bh->b_page);
       mark_page_accessed(bh->b_page);
       __set_page_dirty_nobuffers(bh->b_page);
       unlock_page(bh->b_page);
       set_buffer_uptodate(bh);
       mark_buffer_dirty(bh);
       mark_buffer_async_write(bh);
       unlock_buffer(bh);
       brelse(bh);

What I want is the buffer head not to write synchronously when I do
the brelse, to
coalesce writes on it. It works normally, but this code has some form
of race condition.
The problem is that (in some circumstances, for example while using a
loop device against a file) this makes the fs/buffer.c:603
BUG_ON break. I am using the latest version of the kernel with SMP on a machine
with 4 processors.

Can anybody tell me what am I doing wrong?. I tried some other schemes, like
bit_spin_locking the BH_Uptodate_Lock, but they didn't work either.