Re: [PATCH V6 6/6] SCSI: set block queue at preempt only when SCSI device is put into quiesce

[Bart Van Assche <Bart.VanAssche@wdc.com>]

On Wed, 2017-09-27 at 13:48 +0800, Ming Lei wrote:
> @@ -2928,12 +2929,28 @@ scsi_device_quiesce(struct scsi_device *sdev)
>  {
>  	int err;
>  
> +	/*
> +	 * Simply quiesing SCSI device isn't safe, it is easy
> +	 * to use up requests because all these allocated requests
> +	 * can't be dispatched when device is put in QIUESCE.
> +	 * Then no request can be allocated and we may hang
> +	 * somewhere, such as system suspend/resume.
> +	 *
> +	 * So we set block queue in preempt only first, no new
> +	 * normal request can enter queue any more, and all pending
> +	 * requests are drained once blk_set_preempt_only()
> +	 * returns. Only RQF_PREEMPT is allowed in preempt only mode.
> +	 */
> +	blk_set_preempt_only(sdev->request_queue, true);
> +
>  	mutex_lock(&sdev->state_mutex);
>  	err = scsi_device_set_state(sdev, SDEV_QUIESCE);
>  	mutex_unlock(&sdev->state_mutex);
>  
> -	if (err)
> +	if (err) {
> +		blk_set_preempt_only(sdev->request_queue, false);
>  		return err;
> +	}
>  
>  	scsi_run_queue(sdev->request_queue);
>  	while (atomic_read(&sdev->device_busy)) {
> @@ -2964,6 +2981,8 @@ void scsi_device_resume(struct scsi_device *sdev)
>  	    scsi_device_set_state(sdev, SDEV_RUNNING) == 0)
>  		scsi_run_queue(sdev->request_queue);
>  	mutex_unlock(&sdev->state_mutex);
> +
> +	blk_set_preempt_only(sdev->request_queue, false);

You should have realized yourself that this code is racy. If a request is
allocated just before scsi_device_quiesce() is called and dispatched just
after the device state has been changed into SDEV_QUIESCE then the loop that
waits for all commands to complete will wait forever due to the SCSI prep
function returning BLKPREP_DEFER.

Bart.