Re: [PATCH v4 2/5]nbd: fix might_sleep warning on socket shutdown

[Markus Pargmann]

Hi,

On 2016 M06 30, Thu 14:02:02 CEST 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 | 57
> +++++++++++++++++++++++++++++++++-------------------- 1 file changed, 36
> insertions(+), 21 deletions(-)
> 
> diff --git a/drivers/block/nbd.c b/drivers/block/nbd.c
> index 766c401..e362d44 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 +70,8 @@ struct nbd_device {
>  #if IS_ENABLED(CONFIG_DEBUG_FS)
>  	struct dentry *dbg_dir;
>  #endif
> +	/* This is specifically for calling sock_shutdown, for now. */
> +	struct work_struct ws_shutdown;
>  };
> 
>  #if IS_ENABLED(CONFIG_DEBUG_FS)
> @@ -95,6 +98,8 @@ static int max_part;
>   */
>  static DEFINE_SPINLOCK(nbd_lock);
> 
> +static void nbd_ws_func_shutdown(struct work_struct *);
> +

are you reading all the comments I had?...

At least respond to my comments if you disagree. I still can't see the benefit
of a function signature here if we can avoid it.

>  static inline struct device *nbd_to_dev(struct nbd_device *nbd)
>  {
>  	return disk_to_dev(nbd->disk);
> @@ -172,39 +177,36 @@ 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);
> -
> +	/*
> +	 * Make sure sender thread sees nbd->timedout.
> +	 */
> +	smp_wmb();
> +	schedule_work(&nbd->ws_shutdown);
> +	wake_up(&nbd->waiting_wq);
>  	dev_err(nbd_to_dev(nbd), "Connection timed out, shutting down
> connection\n"); }
> 
> @@ -588,7 +590,11 @@ static int nbd_thread_send(void *data)
>  		spin_unlock_irq(&nbd->queue_lock);
> 
>  		/* handle request */
> -		nbd_handle_req(nbd, req);
> +		if (nbd->timedout) {
> +			req->errors++;
> +			nbd_end_request(nbd, req);
> +		} else
> +			nbd_handle_req(nbd, req);

I already commented on this in the last patch. This is unrelated to the patch.
If you disagree then please tell me why instead of sending the same thing
again.

Also brackets on the else part would be preferred.

Regards,

Markus

>  	}
> 
>  	nbd->task_send = NULL;
> @@ -663,6 +669,7 @@ 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);
>  }
> @@ -797,11 +804,11 @@ 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);
> @@ -857,6 +864,14 @@ 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)