Re: [PATCH 02/12] watchdog: Add the ability to provide data to read

[Guenter Roeck <linux@roeck-us.net>]

On Mon, Aug 19, 2019 at 03:37:01PM -0500, minyard@acm.org wrote:
> From: Corey Minyard <cminyard@mvista.com>
> 
> This is for the read data pretimeout governor.
> 

I am missing an explanation how this is useful and necessary,
and what problem it solves that can not be solved differently.
For my part I don't immediately see the benefits.

Guenter

> Signed-off-by: Corey Minyard <cminyard@mvista.com>
> ---
>  drivers/watchdog/watchdog_core.c |   3 +
>  drivers/watchdog/watchdog_dev.c  | 113 +++++++++++++++++++++++++++++++
>  include/linux/watchdog.h         |   5 ++
>  3 files changed, 121 insertions(+)
> 
> diff --git a/drivers/watchdog/watchdog_core.c b/drivers/watchdog/watchdog_core.c
> index 21e8085b848b..80149ac229fc 100644
> --- a/drivers/watchdog/watchdog_core.c
> +++ b/drivers/watchdog/watchdog_core.c
> @@ -216,6 +216,9 @@ static int __watchdog_register_device(struct watchdog_device *wdd)
>  		return id;
>  	wdd->id = id;
>  
> +	spin_lock_init(&wdd->readlock);
> +	init_waitqueue_head(&wdd->read_q);
> +
>  	ret = watchdog_dev_register(wdd);
>  	if (ret) {
>  		ida_simple_remove(&watchdog_ida, id);
> diff --git a/drivers/watchdog/watchdog_dev.c b/drivers/watchdog/watchdog_dev.c
> index dbd2ad4c9294..8e8304607a8c 100644
> --- a/drivers/watchdog/watchdog_dev.c
> +++ b/drivers/watchdog/watchdog_dev.c
> @@ -44,6 +44,8 @@
>  #include <linux/types.h>	/* For standard types (like size_t) */
>  #include <linux/watchdog.h>	/* For watchdog specific items */
>  #include <linux/uaccess.h>	/* For copy_to_user/put_user/... */
> +#include <linux/poll.h>		/* For poll_table/... */
> +#include <linux/sched/signal.h>	/* For signal_pending */
>  
>  #include <uapi/linux/sched/types.h>	/* For struct sched_param */
>  
> @@ -929,12 +931,120 @@ static int watchdog_release(struct inode *inode, struct file *file)
>  	return 0;
>  }
>  
> +static ssize_t watchdog_read(struct file *file,
> +			     char        __user *buf,
> +			     size_t      count,
> +			     loff_t      *ppos)
> +{
> +	struct watchdog_core_data *wd_data = file->private_data;
> +	struct watchdog_device *wdd;
> +	int err = 0;
> +	wait_queue_entry_t wait;
> +	char dummy = 1;
> +
> +	if (count <= 0)
> +		return 0;
> +
> +	mutex_lock(&wd_data->lock);
> +
> +	wdd = wd_data->wdd;
> +	if (!wdd)
> +		goto done;
> +
> +	/*
> +	 * Reading returns if the pretimeout has gone off, and it only does
> +	 * it once per pretimeout.
> +	 */
> +	spin_lock_irq(&wdd->readlock);
> +	while (!wdd->data_to_read) {
> +		if (file->f_flags & O_NONBLOCK) {
> +			err = -EAGAIN;
> +			goto out;
> +		}
> +
> +		init_waitqueue_entry(&wait, current);
> +		add_wait_queue(&wdd->read_q, &wait);
> +		set_current_state(TASK_INTERRUPTIBLE);
> +		spin_unlock_irq(&wdd->readlock);
> +		schedule();
> +		spin_lock_irq(&wdd->readlock);
> +		remove_wait_queue(&wdd->read_q, &wait);
> +
> +		if (signal_pending(current)) {
> +			err = -ERESTARTSYS;
> +			goto out;
> +		}
> +	}
> +	dummy = wdd->data_to_read;
> +	wdd->data_to_read = 0;
> +
> + out:
> +	spin_unlock_irq(&wdd->readlock);
> +
> +	if (err == 0) {
> +		if (copy_to_user(buf, &dummy, 1))
> +			err = -EFAULT;
> +		else
> +			err = 1;
> +	}
> +
> + done:
> +	mutex_unlock(&wd_data->lock);
> +
> +	return err;
> +}
> +
> +static __poll_t watchdog_poll(struct file *file, poll_table *wait)
> +{
> +	struct watchdog_core_data *wd_data = file->private_data;
> +	struct watchdog_device *wdd;
> +	__poll_t mask = 0;
> +
> +	mutex_lock(&wd_data->lock);
> +
> +	wdd = wd_data->wdd;
> +	if (!wdd)
> +		goto done;
> +
> +	poll_wait(file, &wdd->read_q, wait);
> +
> +	spin_lock_irq(&wdd->readlock);
> +	if (wdd->data_to_read)
> +		mask |= (EPOLLIN | EPOLLRDNORM);
> +	spin_unlock_irq(&wdd->readlock);
> +
> +done:
> +	mutex_unlock(&wd_data->lock);
> +	return mask;
> +}
> +
> +static int watchdog_fasync(int fd, struct file *file, int on)
> +{
> +	struct watchdog_core_data *wd_data = file->private_data;
> +	struct watchdog_device *wdd;
> +	int err = -ENODEV;
> +
> +	mutex_lock(&wd_data->lock);
> +
> +	wdd = wd_data->wdd;
> +	if (!wdd)
> +		goto done;
> +
> +	err = fasync_helper(fd, file, on, &wdd->fasync_q);
> +done:
> +	mutex_unlock(&wd_data->lock);
> +	return err;
> +}
> +
>  static const struct file_operations watchdog_fops = {
>  	.owner		= THIS_MODULE,
>  	.write		= watchdog_write,
>  	.unlocked_ioctl	= watchdog_ioctl,
>  	.open		= watchdog_open,
>  	.release	= watchdog_release,
> +	.read		= watchdog_read,
> +	.poll		= watchdog_poll,
> +	.fasync		= watchdog_fasync,
>  };
>  
>  static struct miscdevice watchdog_miscdev = {
> @@ -970,6 +1080,9 @@ static int watchdog_cdev_register(struct watchdog_device *wdd, dev_t devno)
>  	if (IS_ERR_OR_NULL(watchdog_kworker))
>  		return -ENODEV;
>  
> +	spin_lock_init(&wdd->readlock);
> +	init_waitqueue_head(&wdd->read_q);
> +
>  	kthread_init_work(&wd_data->work, watchdog_ping_work);
>  	hrtimer_init(&wd_data->timer, CLOCK_MONOTONIC, HRTIMER_MODE_REL);
>  	wd_data->timer.function = watchdog_timer_expired;
> diff --git a/include/linux/watchdog.h b/include/linux/watchdog.h
> index 417d9f37077a..e34501a822f0 100644
> --- a/include/linux/watchdog.h
> +++ b/include/linux/watchdog.h
> @@ -117,6 +117,11 @@ struct watchdog_device {
>  #define WDOG_HW_RUNNING		3	/* True if HW watchdog running */
>  #define WDOG_STOP_ON_UNREGISTER	4	/* Should be stopped on unregister */
>  	struct list_head deferred;
> +
> +	spinlock_t readlock;
> +	bool data_to_read;
> +	struct wait_queue_head read_q;
> +	struct fasync_struct *fasync_q;
>  };
>  
>  #define WATCHDOG_NOWAYOUT		IS_BUILTIN(CONFIG_WATCHDOG_NOWAYOUT)
> -- 
> 2.17.1
>