Re: [PATCH 2/2] loop: Perform __loop_clr_fd() after disk->open_mutex is dropped.

[Bart Van Assche <bvanassche@acm.org>]

On 9/9/26 3:49 AM, Tetsuo Handa wrote:
> -static void __loop_clr_fd(struct loop_device *lo)
> +static void __loop_clr_fd(struct work_struct *work)
>   {
> +	struct loop_device *lo = container_of(work, struct loop_device, lo_clr_work);
> +	struct gendisk *disk = lo->lo_disk;
>   	struct queue_limits lim;
>   	struct file *filp;
>   	gfp_t gfp = lo->old_gfp_mask;
>   	int err;
>   
> +	/* Step 1: Flush all outstanding I/O, without open_mutex held. */
> +	/*
> +	 * Since loop_queue_rq() is called with RCU read lock, this synchronize_rcu()
> +	 * makes sure that no more queue_work() calls are made from loop_queue_work()
> +	 * from loop_queue_rq(). Subsequent loop_queue_rq() calls which are made after
> +	 * this synchronize_rcu() returned shall see lo->lo_state != Lo_bound and
> +	 * return with BLK_STS_IOERR.
> +	 */
> +	synchronize_rcu();
> +	/*
> +	 * This drain_workqueue() makes sure that no more loop_handle_cmd() calls are
> +	 * made from loop_process_work() from loop_workfn()/loop_rootcg_workfn().
> +	 */
> +	drain_workqueue(lo->workqueue);
> +	/*
> +	 * This blk_mq_freeze_queue() waits for completion of all outstanding I/O
> +	 * which has been scheduled via loop_queue_rq(), by waiting for q_usage_counter
> +	 * to reach 0. Since the lo->lo_state != Lo_bound check in loop_queue_rq()
> +	 * guarantees that no more new I/O requests are made, we can call
> +	 * blk_mq_unfreeze_queue() immediately after blk_mq_freeze_queue() returns.
> +	 */
> +	blk_mq_unfreeze_queue(lo->lo_queue, blk_mq_freeze_queue(lo->lo_queue));
> +
> +	/* Step 2: Perform remaining cleanup, with open_mutex held. */
> +	mutex_lock(&disk->open_mutex);
> +	WARN_ON_ONCE(lo->lo_state != Lo_rundown);
__loop_clr_fd() is queued from the lo_post_release() callback and hence
may be called concurrently with or after another thread has called
bdev_open(). Hence, lo->lo->state should be checked instead of assuming
that it equals Lo_rundown.

Thanks,

Bart.