Re: [PATCH v3 1/3]nbd: fix might_sleep warning on socket shutdown

[Markus Pargmann]

On Friday 24 June 2016 13:09:34 Pranay Kr. Srivastava wrote:
> spinlocked ranges should be small and not contain calls into huge
> subfunctions. Fix my mistake and just get the pointer to the socket
> instead of doing everything with spinlock held.
> 
> Reported-by: Mikulas Patocka <mikulas@xxxxxxxxxxxxx>
> Signed-off-by: Markus Pargmann <mpa@xxxxxxxxxxxxxx>
> 
> Changelog:
> Pranay Kr. Srivastava<pranjas@xxxxxxxxx>:
> 
> 1) Use spin_lock instead of irq version for sock_shutdown.
> 
> 2) Use system work queue to actually trigger the shutdown of
>    socket. This solves the issue when kernel_sendmsg is currently
>    blocked while a timeout occurs.
> 
> Signed-off-by: Pranay Kr. Srivastava <pranjas@xxxxxxxxx>
> ---
>  drivers/block/nbd.c | 69 ++++++++++++++++++++++++++++++++++-------------------
>  1 file changed, 44 insertions(+), 25 deletions(-)
> 
> diff --git a/drivers/block/nbd.c b/drivers/block/nbd.c
> index 56f7f5d..586d946 100644
> --- a/drivers/block/nbd.c
> +++ b/drivers/block/nbd.c
> @@ -39,6 +39,7 @@
>  #include <asm/types.h>
>  
>  #include <linux/nbd.h>
> +#include <linux/workqueue.h>
>  
>  struct nbd_device {
>  	u32 flags;
> @@ -69,6 +70,10 @@ struct nbd_device {
>  #if IS_ENABLED(CONFIG_DEBUG_FS)
>  	struct dentry *dbg_dir;
>  #endif
> +	/*
> +	*This is specifically for calling sock_shutdown, for now.
> +	*/

Please fix the indentation of this comment.

> +	struct work_struct ws_shutdown;
>  };
>  
>  #if IS_ENABLED(CONFIG_DEBUG_FS)
> @@ -95,6 +100,11 @@ static int max_part;
>   */
>  static DEFINE_SPINLOCK(nbd_lock);
>  
> +/*
> + * Shutdown function for nbd_dev work struct.
> + */
> +static void nbd_ws_func_shutdown(struct work_struct *);

You could as well put the function implementation here. No need for a
function signature.

> +
>  static inline struct device *nbd_to_dev(struct nbd_device *nbd)
>  {
>  	return disk_to_dev(nbd->disk);
> @@ -172,39 +182,35 @@ static void nbd_end_request(struct nbd_device *nbd, struct request *req)
>   */
>  static void sock_shutdown(struct nbd_device *nbd)
>  {
> -	spin_lock_irq(&nbd->sock_lock);
> -
> -	if (!nbd->sock) {
> -		spin_unlock_irq(&nbd->sock_lock);
> -		return;
> -	}
> +	struct socket *sock;
>  
> -	dev_warn(disk_to_dev(nbd->disk), "shutting down socket\n");
> -	kernel_sock_shutdown(nbd->sock, SHUT_RDWR);
> -	sockfd_put(nbd->sock);
> +	spin_lock(&nbd->sock_lock);
> +	sock = nbd->sock;
>  	nbd->sock = NULL;
> -	spin_unlock_irq(&nbd->sock_lock);
> +	spin_unlock(&nbd->sock_lock);
> +
> +	if (!sock)
> +		return;
>  
>  	del_timer(&nbd->timeout_timer);
> +	dev_warn(disk_to_dev(nbd->disk), "shutting down socket\n");
> +	kernel_sock_shutdown(sock, SHUT_RDWR);
> +	sockfd_put(sock);
>  }
>  
>  static void nbd_xmit_timeout(unsigned long arg)
>  {
>  	struct nbd_device *nbd = (struct nbd_device *)arg;
> -	unsigned long flags;
>  
>  	if (list_empty(&nbd->queue_head))
>  		return;
> -
> -	spin_lock_irqsave(&nbd->sock_lock, flags);
> -
>  	nbd->timedout = true;
> -
> -	if (nbd->sock)
> -		kernel_sock_shutdown(nbd->sock, SHUT_RDWR);
> -
> -	spin_unlock_irqrestore(&nbd->sock_lock, flags);
> -
> +	schedule_work(&nbd->ws_shutdown);
> +	/*
> +	 * Make sure sender thread sees nbd->timedout.
> +	 */
> +	smp_wmb();
> +	wake_up(&nbd->waiting_wq);
>  	dev_err(nbd_to_dev(nbd), "Connection timed out, shutting down connection\n");
>  }
>  
> @@ -574,8 +580,8 @@ static int nbd_thread_send(void *data)
>  	while (!kthread_should_stop() || !list_empty(&nbd->waiting_queue)) {
>  		/* wait for something to do */
>  		wait_event_interruptible(nbd->waiting_wq,
> -					 kthread_should_stop() ||
> -					 !list_empty(&nbd->waiting_queue));
> +				kthread_should_stop() ||
> +				!list_empty(&nbd->waiting_queue));

This is unrelated, please remove.

>  
>  		/* extract request */
>  		if (list_empty(&nbd->waiting_queue))
> @@ -583,12 +589,16 @@ static int nbd_thread_send(void *data)
>  
>  		spin_lock_irq(&nbd->queue_lock);
>  		req = list_entry(nbd->waiting_queue.next, struct request,
> -				 queuelist);
> +				queuelist);

Unrelated as well.

>  		list_del_init(&req->queuelist);
>  		spin_unlock_irq(&nbd->queue_lock);
>  
> -		/* handle request */

Unrelated.

>  		nbd_handle_req(nbd, req);
> +		if (nbd->timedout) {
> +			req->errors++;
> +			nbd_end_request(nbd, req);
> +		} else
> +			nbd_handle_req(nbd, req);

This does not change anything to avoid the spinlock sock_shutdown issue.
Please split in a separate patch.

Also this is already handled in nbd_handle_req(). For !nbd->sock the
same code is executed.


Otherwise the patch looks good.

Best Regards,

Markus

>  	}
>  
>  	nbd->task_send = NULL;
> @@ -668,6 +678,7 @@ static void nbd_reset(struct nbd_device *nbd)
>  	set_capacity(nbd->disk, 0);
>  	nbd->flags = 0;
>  	nbd->xmit_timeout = 0;
> +	INIT_WORK(&nbd->ws_shutdown, nbd_ws_func_shutdown);
>  	queue_flag_clear_unlocked(QUEUE_FLAG_DISCARD, nbd->disk->queue);
>  	del_timer_sync(&nbd->timeout_timer);
>  }
> @@ -802,11 +813,11 @@ static int __nbd_ioctl(struct block_device *bdev, struct nbd_device *nbd,
>  		error = nbd_thread_recv(nbd, bdev);
>  		nbd_dev_dbg_close(nbd);
>  		kthread_stop(thread);
> +		sock_shutdown(nbd);
>  
>  		mutex_lock(&nbd->tx_lock);
>  		nbd->task_recv = NULL;
>  
> -		sock_shutdown(nbd);
>  		nbd_clear_que(nbd);
>  		kill_bdev(bdev);
>  		nbd_bdev_reset(bdev);
> @@ -862,6 +873,14 @@ static const struct block_device_operations nbd_fops = {
>  	.compat_ioctl =	nbd_ioctl,
>  };
>  
> +static void nbd_ws_func_shutdown(struct work_struct *ws_nbd)
> +{
> +	struct nbd_device *nbd_dev = container_of(ws_nbd, struct nbd_device,
> +			ws_shutdown);
> +
> +	sock_shutdown(nbd_dev);
> +}
> +
>  #if IS_ENABLED(CONFIG_DEBUG_FS)
>  
>  static int nbd_dbg_tasks_show(struct seq_file *s, void *unused)
> 

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