Re: [RFC PATCH] blkcg: prevent priority inversion problem during sync()

[Andrea Righi <righi.andrea@gmail.com>]

On Sat, Feb 09, 2019 at 01:06:33PM +0100, Andrea Righi wrote:
...
> +/**
> + * blkcg_wb_waiters_on_bdi - check for writeback waiters on a block device
> + * @bdi: block device to check
> + *
> + * Return true if any other blkcg is waiting for writeback on the target block
> + * device, false otherwise.
> + */
> +bool blkcg_wb_waiters_on_bdi(struct backing_dev_info *bdi)
> +{
> +	struct blkcg *blkcg, *curr_blkcg;
> +	bool ret = false;
> +
> +	if (unlikely(!bdi))
> +		return false;
> +
> +	rcu_read_lock();
> +	curr_blkcg = css_to_blkcg(task_css(current, io_cgrp_id));

Sorry, the logic is messed up here. We shouldn't get curr_blkcg from the
current task, because during writeback throttling the context is
obviously not the current task.

I'll post a new patch soon.

> +	list_for_each_entry_rcu(blkcg, &bdi->cgwb_waiters, cgwb_wait_node)
> +		if (blkcg != curr_blkcg) {
> +			ret = true;
> +			break;
> +		}
> +	rcu_read_unlock();
> +
> +	return ret;
> +}

-Andrea