linux-kernel - Re: [PATCH] Block - Honour barrier requests in loop driver

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Date:	Tue, 17 Mar 2009 05:19:26 -0400
From:	Christoph Hellwig <hch@...radead.org>
To:	Nikanth Karthikesan <knikanth@...e.de>
Cc:	Jens Axboe <jens.axboe@...cle.com>,
	Gerd Hoffmann <kraxel@...hat.com>,
	Constantine Sapuntzakis <csapuntz@...il.com>,
	Miklos Szeredi <miklos@...redi.hu>, nikanth@...il.com,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] Block - Honour barrier requests in loop driver

On Tue, Mar 17, 2009 at 02:17:16PM +0530, Nikanth Karthikesan wrote:
> +static int sync_file(struct file *file)
> +{
> +	struct address_space *mapping;
> +	int ret;
> +
> +	if (!file->f_op || !file->f_op->fsync)
> +		return -EOPNOTSUPP;
> +
> +	mapping = file->f_mapping;
> +
> +	ret = filemap_fdatawrite(mapping);
> +	if (!ret) {
> +		int ret2;
> +
> +		mutex_lock(&mapping->host->i_mutex);
> +		ret = file->f_op->fsync(file, file->f_dentry, 1);
> +		mutex_unlock(&mapping->host->i_mutex);
> +
> +		ret2 = filemap_fdatawait(mapping);
> +		if (!ret)
> +			ret = ret2;

Please use vfs_fsync.

> +	int barrier = bio_barrier(bio);
> +
> +	if (barrier) {
> +		ret = sync_file(lo->lo_backing_file);
> +		if (unlikely(ret))
> +			goto out;
> +	}
>  
>  	pos = ((loff_t) bio->bi_sector << 9) + lo->lo_offset;
> +	if (bio_rw(bio) == WRITE) {
>  		ret = lo_send(lo, bio, pos);
> +		if (barrier && !ret)
> +			ret = sync_file(lo->lo_backing_file);
> +	} else
>  		ret = lo_receive(lo, bio, lo->lo_blocksize, pos);
> +
> +out:
>  	return ret;

We only use barrier requests for reads, which this code relies on for
the second sync, too.  So just move the whole thing into one if block,
ala:


 	pos = ((loff_t) bio->bi_sector << 9) + lo->lo_offset;
	if (bio_rw(bio) == WRITE) {
		int barrier = bio_barrier(bio);

		if (barrier) {
			ret = sync_file(lo->lo_backing_file);
			if (unlikely(ret))
				goto out;
		}

 		ret = lo_send(lo, bio, pos);
		if (ret)
			goto out;

		if (barrier)
			ret = sync_file(lo->lo_backing_file);
	} else
		ret = lo_receive(lo, bio, lo->lo_blocksize, pos);

out:
 	return ret;

You also should advertise the barrier capability with a queue flag.
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