Re: [PATCH V2 4/8] ublk: move device reset into ublk_ch_release()

[Uday Shankar <ushankar@purestorage.com>]

On Wed, Apr 16, 2025 at 11:54:38AM +0800, Ming Lei wrote:
> ublk_ch_release() is called after ublk char device is closed, when all
> uring_cmd are done, so it is perfect fine to move ublk device reset to
> ublk_ch_release() from ublk_ctrl_start_recovery().
> 
> This way can avoid to grab the exiting daemon task_struct too long.
> 
> However, reset of the following ublk IO flags has to be moved until ublk
> io_uring queues are ready:
> 
> - ubq->canceling
> 
> For requeuing IO in case of ublk_nosrv_dev_should_queue_io() before device
> is recovered
> 
> - ubq->fail_io
> 
> For failing IO in case of UBLK_F_USER_RECOVERY_FAIL_IO before device is
> recovered
> 
> - ublk_io->flags
> 
> For preventing using io->cmd
> 
> With this way, recovery is simplified a lot.
> 
> Signed-off-by: Ming Lei <ming.lei@redhat.com>
> ---
>  drivers/block/ublk_drv.c | 121 +++++++++++++++++++++++----------------
>  1 file changed, 72 insertions(+), 49 deletions(-)
> 
> diff --git a/drivers/block/ublk_drv.c b/drivers/block/ublk_drv.c
> index a479969fd77e..1fe39cf85b2f 100644
> --- a/drivers/block/ublk_drv.c
> +++ b/drivers/block/ublk_drv.c
> @@ -1074,7 +1074,7 @@ static inline struct ublk_uring_cmd_pdu *ublk_get_uring_cmd_pdu(
>  
>  static inline bool ubq_daemon_is_dying(struct ublk_queue *ubq)
>  {
> -	return ubq->ubq_daemon->flags & PF_EXITING;
> +	return !ubq->ubq_daemon || ubq->ubq_daemon->flags & PF_EXITING;
>  }
>  
>  /* todo: handle partial completion */
> @@ -1470,6 +1470,37 @@ static const struct blk_mq_ops ublk_mq_ops = {
>  	.timeout	= ublk_timeout,
>  };
>  
> +static void ublk_queue_reinit(struct ublk_device *ub, struct ublk_queue *ubq)
> +{
> +	int i;
> +
> +	/* All old ioucmds have to be completed */
> +	ubq->nr_io_ready = 0;
> +
> +	/*
> +	 * old daemon is PF_EXITING, put it now
> +	 *
> +	 * It could be NULL in case of closing one quisced device.
> +	 */
> +	if (ubq->ubq_daemon)
> +		put_task_struct(ubq->ubq_daemon);
> +	/* We have to reset it to NULL, otherwise ub won't accept new FETCH_REQ */
> +	ubq->ubq_daemon = NULL;
> +	ubq->timeout = false;
> +
> +	for (i = 0; i < ubq->q_depth; i++) {
> +		struct ublk_io *io = &ubq->ios[i];
> +
> +		/*
> +		 * UBLK_IO_FLAG_CANCELED is kept for avoiding to touch
> +		 * io->cmd
> +		 */
> +		io->flags &= UBLK_IO_FLAG_CANCELED;
> +		io->cmd = NULL;
> +		io->addr = 0;
> +	}
> +}
> +
>  static int ublk_ch_open(struct inode *inode, struct file *filp)
>  {
>  	struct ublk_device *ub = container_of(inode->i_cdev,
> @@ -1481,10 +1512,26 @@ static int ublk_ch_open(struct inode *inode, struct file *filp)
>  	return 0;
>  }
>  
> +static void ublk_reset_ch_dev(struct ublk_device *ub)
> +{
> +	int i;
> +
> +	for (i = 0; i < ub->dev_info.nr_hw_queues; i++)
> +		ublk_queue_reinit(ub, ublk_get_queue(ub, i));
> +
> +	/* set to NULL, otherwise new ubq_daemon cannot mmap the io_cmd_buf */
> +	ub->mm = NULL;
> +	ub->nr_queues_ready = 0;
> +	ub->nr_privileged_daemon = 0;
> +}
> +
>  static int ublk_ch_release(struct inode *inode, struct file *filp)
>  {
>  	struct ublk_device *ub = filp->private_data;
>  
> +	/* all uring_cmd has been done now, reset device & ubq */
> +	ublk_reset_ch_dev(ub);

Can we take the ub->mutex here? I can't see a concrete data race but I
feel there is lots of potential for one since the device still may be
concurrently accessed via ctrl commands.

> +
>  	clear_bit(UB_STATE_OPEN, &ub->state);
>  	return 0;
>  }
> @@ -1831,6 +1878,24 @@ static void ublk_nosrv_work(struct work_struct *work)
>  	ublk_cancel_dev(ub);
>  }
>  
> +/* reset ublk io_uring queue & io flags */
> +static void ublk_reset_io_flags(struct ublk_device *ub)
> +{
> +	int i, j;
> +
> +	for (i = 0; i < ub->dev_info.nr_hw_queues; i++) {
> +		struct ublk_queue *ubq = ublk_get_queue(ub, i);
> +
> +		/* UBLK_IO_FLAG_CANCELED can be cleared now */
> +		spin_lock(&ubq->cancel_lock);
> +		for (j = 0; j < ubq->q_depth; j++)
> +			ubq->ios[j].flags &= ~UBLK_IO_FLAG_CANCELED;
> +		spin_unlock(&ubq->cancel_lock);
> +		ubq->canceling = false;
> +		ubq->fail_io = false;
> +	}
> +}
> +
>  /* device can only be started after all IOs are ready */
>  static void ublk_mark_io_ready(struct ublk_device *ub, struct ublk_queue *ubq)
>  	__must_hold(&ub->mutex)
> @@ -1844,8 +1909,12 @@ static void ublk_mark_io_ready(struct ublk_device *ub, struct ublk_queue *ubq)
>  		if (capable(CAP_SYS_ADMIN))
>  			ub->nr_privileged_daemon++;
>  	}
> -	if (ub->nr_queues_ready == ub->dev_info.nr_hw_queues)
> +
> +	if (ub->nr_queues_ready == ub->dev_info.nr_hw_queues) {
> +		/* now we are ready for handling ublk io request */
> +		ublk_reset_io_flags(ub);
>  		complete_all(&ub->completion);
> +	}
>  }
>  
>  static void ublk_handle_need_get_data(struct ublk_device *ub, int q_id,
> @@ -2954,41 +3023,14 @@ static int ublk_ctrl_set_params(struct ublk_device *ub,
>  	return ret;
>  }
>  
> -static void ublk_queue_reinit(struct ublk_device *ub, struct ublk_queue *ubq)
> -{
> -	int i;
> -
> -	WARN_ON_ONCE(!(ubq->ubq_daemon && ubq_daemon_is_dying(ubq)));
> -
> -	/* All old ioucmds have to be completed */
> -	ubq->nr_io_ready = 0;
> -	/* old daemon is PF_EXITING, put it now */
> -	put_task_struct(ubq->ubq_daemon);
> -	/* We have to reset it to NULL, otherwise ub won't accept new FETCH_REQ */
> -	ubq->ubq_daemon = NULL;
> -	ubq->timeout = false;
> -
> -	for (i = 0; i < ubq->q_depth; i++) {
> -		struct ublk_io *io = &ubq->ios[i];
> -
> -		/* forget everything now and be ready for new FETCH_REQ */
> -		io->flags = 0;
> -		io->cmd = NULL;
> -		io->addr = 0;
> -	}
> -}
> -
>  static int ublk_ctrl_start_recovery(struct ublk_device *ub,
>  		const struct ublksrv_ctrl_cmd *header)
>  {
>  	int ret = -EINVAL;
> -	int i;
>  
>  	mutex_lock(&ub->mutex);
>  	if (ublk_nosrv_should_stop_dev(ub))
>  		goto out_unlock;
> -	if (!ub->nr_queues_ready)
> -		goto out_unlock;
>  	/*
>  	 * START_RECOVERY is only allowd after:
>  	 *
> @@ -3012,12 +3054,6 @@ static int ublk_ctrl_start_recovery(struct ublk_device *ub,
>  		goto out_unlock;
>  	}
>  	pr_devel("%s: start recovery for dev id %d.\n", __func__, header->dev_id);
> -	for (i = 0; i < ub->dev_info.nr_hw_queues; i++)
> -		ublk_queue_reinit(ub, ublk_get_queue(ub, i));
> -	/* set to NULL, otherwise new ubq_daemon cannot mmap the io_cmd_buf */
> -	ub->mm = NULL;
> -	ub->nr_queues_ready = 0;
> -	ub->nr_privileged_daemon = 0;
>  	init_completion(&ub->completion);
>  	ret = 0;
>   out_unlock:
> @@ -3030,7 +3066,6 @@ static int ublk_ctrl_end_recovery(struct ublk_device *ub,
>  {
>  	int ublksrv_pid = (int)header->data[0];
>  	int ret = -EINVAL;
> -	int i;
>  
>  	pr_devel("%s: Waiting for new ubq_daemons(nr: %d) are ready, dev id %d...\n",
>  			__func__, ub->dev_info.nr_hw_queues, header->dev_id);
> @@ -3050,22 +3085,10 @@ static int ublk_ctrl_end_recovery(struct ublk_device *ub,
>  		goto out_unlock;
>  	}
>  	ub->dev_info.ublksrv_pid = ublksrv_pid;
> +	ub->dev_info.state = UBLK_S_DEV_LIVE;
>  	pr_devel("%s: new ublksrv_pid %d, dev id %d\n",
>  			__func__, ublksrv_pid, header->dev_id);
> -
> -	blk_mq_quiesce_queue(ub->ub_disk->queue);
> -	ub->dev_info.state = UBLK_S_DEV_LIVE;
> -	for (i = 0; i < ub->dev_info.nr_hw_queues; i++) {
> -		struct ublk_queue *ubq = ublk_get_queue(ub, i);
> -
> -		ubq->canceling = false;
> -		ubq->fail_io = false;
> -	}
> -	blk_mq_unquiesce_queue(ub->ub_disk->queue);
> -	pr_devel("%s: queue unquiesced, dev id %d.\n",
> -			__func__, header->dev_id);
>  	blk_mq_kick_requeue_list(ub->ub_disk->queue);
> -
>  	ret = 0;
>   out_unlock:
>  	mutex_unlock(&ub->mutex);
> -- 
> 2.47.0
>