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

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Date:   Sat, 9 Feb 2019 14:52:31 +0100
From:   Andrea Righi <righi.andrea@...il.com>
To:     Josef Bacik <josef@...icpanda.com>
Cc:     Paolo Valente <paolo.valente@...aro.org>,
        Tejun Heo <tj@...nel.org>, Li Zefan <lizefan@...wei.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Jens Axboe <axboe@...nel.dk>, Vivek Goyal <vgoyal@...hat.com>,
        Dennis Zhou <dennis@...nel.org>, cgroups@...r.kernel.org,
        linux-block@...r.kernel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH] blkcg: prevent priority inversion problem during
 sync()

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