Re: [PATCH 1/3] dio: track and serialise unaligned direct IO

[Jeff Moyer <jmoyer@redhat.com>]

Dave Chinner <david@fromorbit.com> writes:

> +struct dio_zero_block {
> +	struct list_head dio_list;	/* list of io in progress */
> +	sector_t	zero_block;	/* block being zeroed */
> +	struct dio	*dio;		/* owner dio */
> +	wait_queue_head_t wq;		/* New IO block here */
                      New IOs block here, or new IO blocks here?

> +/*
> + * Add a filesystem block to the list of blocks we are tracking.
> + */
> +static void
> +dio_start_zero_block(struct dio *dio, sector_t zero_block)
> +{
> +	struct dio_zero_block *zb;
> +
> +	zb = kmalloc(sizeof(*zb), GFP_NOIO);
> +	if (!zb)
> +		return;
> +	INIT_LIST_HEAD(&zb->dio_list);
> +	init_waitqueue_head(&zb->wq);
> +	zb->zero_block = zero_block;
> +	zb->dio = dio;
> +	atomic_set(&zb->ref, 1);
> +
> +	spin_lock(&dio_zero_block_lock);
> +	list_add(&zb->dio_list, &dio_zero_block_list);
> +	spin_unlock(&dio_zero_block_lock);

What protects from two processes getting here at the same time, and
hence adding the same block to the list?  i_mutex?

> @@ -1210,6 +1343,13 @@ __blockdev_direct_IO(int rw, struct kiocb *iocb, struct inode *inode,
>  	 */
>  	memset(dio, 0, offsetof(struct dio, pages));
>  
> +	/*
> +	 * zero_blocks need to initialised to largeѕt value to avoid
                              ^ be