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Date: Wed, 30 Aug 2006 11:35:44 -0500
From: "Gorka Guardiola" <paurealkml@gmail.com>
To: linux-kernel@vger.kernel.org
Subject: Buffer head async write
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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 it didn't work either.