Re: [PATCH v3 11/30] staging: unisys: visorbus: use kernel timer instead of workqueue

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On Sat, Jun 04, 2016 at 01:27:11PM -0400, David Kershner wrote:
> From: Tim Sell <Timothy.Sell@xxxxxxxxxx>
> 
> A kernel timer is now used as the vehicle to periodically call the
> channel_interrupt function of registered visor drivers, instead of a
> workqueue.
> 
> This simplifies a lot of things by making periodic_work.c and
> periodic_work.h no longer necessary.  This change also means that the
> channel_interrupt() callbacks registered by visor drivers (via
> visorbus_register_visor_driver()) will now be called in atomic context
> (i.e., canNOT sleep) rather than kernel thread context (CAN sleep).
> Fortunately this did NOT necessitate any change to the existing
> channel_interrupt() callbacks, because none of them ever perform any
> operations that would be invalid in atomic context.
> 
> Signed-off-by: Tim Sell <Timothy.Sell@xxxxxxxxxx>
> Signed-off-by: David Kershner <david.kershner@xxxxxxxxxx>
> ---
>  drivers/staging/unisys/include/visorbus.h       | 10 +++--
>  drivers/staging/unisys/visorbus/visorbus_main.c | 54 +++++++------------------
>  2 files changed, 21 insertions(+), 43 deletions(-)
> 
> diff --git a/drivers/staging/unisys/include/visorbus.h b/drivers/staging/unisys/include/visorbus.h
> index 9baf1ec..9bb88bb 100644
> --- a/drivers/staging/unisys/include/visorbus.h
> +++ b/drivers/staging/unisys/include/visorbus.h
> @@ -34,8 +34,9 @@
>  #include <linux/poll.h>
>  #include <linux/kernel.h>
>  #include <linux/uuid.h>
> +#include <linux/seq_file.h>
> +#include <linux/slab.h>
>  
> -#include "periodic_work.h"
>  #include "channel.h"
>  
>  struct visor_driver;
> @@ -126,8 +127,8 @@ struct visor_driver {
>   * device:			Device struct meant for use by the bus driver
>   *				only.
>   * list_all:			Used by the bus driver to enumerate devices.
> - * periodic_work:		Device work queue. Private use by bus driver
> - *				only.
> + * timer:		        Timer fired periodically to do interrupt-type
> + *				activity.
>   * being_removed:		Indicates that the device is being removed from
>   *				the bus. Private bus driver use only.
>   * visordriver_callback_lock:	Used by the bus driver to lock when handling
> @@ -157,7 +158,8 @@ struct visor_device {
>  	/* These fields are for private use by the bus driver only. */
>  	struct device device;
>  	struct list_head list_all;
> -	struct periodic_work *periodic_work;
> +	struct timer_list timer;
> +	bool timer_active;
>  	bool being_removed;
>  	struct semaphore visordriver_callback_lock;
>  	bool pausing;
> diff --git a/drivers/staging/unisys/visorbus/visorbus_main.c b/drivers/staging/unisys/visorbus/visorbus_main.c
> index ebdd5de..e98e720 100644
> --- a/drivers/staging/unisys/visorbus/visorbus_main.c
> +++ b/drivers/staging/unisys/visorbus/visorbus_main.c
> @@ -19,7 +19,6 @@
>  #include "visorbus.h"
>  #include "visorbus_private.h"
>  #include "version.h"
> -#include "periodic_work.h"
>  #include "vbuschannel.h"
>  #include "guestlinuxdebug.h"
>  #include "vmcallinterface.h"
> @@ -116,7 +115,6 @@ struct bus_type visorbus_type = {
>  	.bus_groups = visorbus_bus_groups,
>  };
>  
> -static struct workqueue_struct *periodic_dev_workqueue;
>  static long long bus_count;	/** number of bus instances */
>  					/** ever-increasing */
>  
> @@ -222,10 +220,6 @@ visorbus_release_device(struct device *xdev)
>  {
>  	struct visor_device *dev = to_visor_device(xdev);
>  
> -	if (dev->periodic_work) {
> -		visor_periodic_work_destroy(dev->periodic_work);
> -		dev->periodic_work = NULL;
> -	}
>  	if (dev->visorchannel) {
>  		visorchannel_destroy(dev->visorchannel);
>  		dev->visorchannel = NULL;
> @@ -530,35 +524,36 @@ unregister_driver_attributes(struct visor_driver *drv)
>  }
>  
>  static void
> -dev_periodic_work(void *xdev)
> +dev_periodic_work(unsigned long __opaque)
>  {
> -	struct visor_device *dev = xdev;
> +	struct visor_device *dev = (struct visor_device *)__opaque;
>  	struct visor_driver *drv = to_visor_driver(dev->device.driver);
>  
> -	down(&dev->visordriver_callback_lock);
>  	if (drv->channel_interrupt)
>  		drv->channel_interrupt(dev);
visorinput_channel_interrupt uses down_write (and other channel_interrupt
methods may do the same).  While its legal to use semaphores in interrupt
context, you need to convert that/those calls to the trylock variants and handle
a failed lock to avoid blocking when in interrupt context, which moving to a
timer handler puts you in.

> -	up(&dev->visordriver_callback_lock);
> -	if (!visor_periodic_work_nextperiod(dev->periodic_work))
> -		put_device(&dev->device);
> +	mod_timer(&dev->timer, jiffies + POLLJIFFIES_NORMALCHANNEL);
>  }
>  
>  static void
>  dev_start_periodic_work(struct visor_device *dev)
>  {
> -	if (dev->being_removed)
> +	if (dev->being_removed || dev->timer_active)
>  		return;
>  	/* now up by at least 2 */
>  	get_device(&dev->device);
> -	if (!visor_periodic_work_start(dev->periodic_work))
> -		put_device(&dev->device);
> +	dev->timer.expires = jiffies + POLLJIFFIES_NORMALCHANNEL;
> +	add_timer(&dev->timer);
> +	dev->timer_active = true;
>  }
>  
>  static void
>  dev_stop_periodic_work(struct visor_device *dev)
>  {
> -	if (visor_periodic_work_stop(dev->periodic_work))
> -		put_device(&dev->device);
> +	if (!dev->timer_active)
> +		return;
> +	del_timer_sync(&dev->timer);
> +	dev->timer_active = false;
> +	put_device(&dev->device);
>  }
>  
>  /** This is called automatically upon adding a visor_device (device_add), or
> @@ -776,17 +771,9 @@ create_visor_device(struct visor_device *dev)
>  	dev->device.release = visorbus_release_device;
>  	/* keep a reference just for us (now 2) */
>  	get_device(&dev->device);
> -	dev->periodic_work =
> -		visor_periodic_work_create(POLLJIFFIES_NORMALCHANNEL,
> -					   periodic_dev_workqueue,
> -					   dev_periodic_work,
> -					   dev, dev_name(&dev->device));
> -	if (!dev->periodic_work) {
> -		POSTCODE_LINUX_3(DEVICE_CREATE_FAILURE_PC, chipset_dev_no,
> -				 DIAG_SEVERITY_ERR);
> -		err = -EINVAL;
> -		goto err_put;
> -	}
> +	init_timer(&dev->timer);
> +	dev->timer.data = (unsigned long)(dev);
> +	dev->timer.function = dev_periodic_work;
>  
>  	/* bus_id must be a unique name with respect to this bus TYPE
>  	 * (NOT bus instance).  That's why we need to include the bus
> @@ -1268,13 +1255,6 @@ visorbus_init(void)
>  		goto error;
>  	}
>  
> -	periodic_dev_workqueue = create_singlethread_workqueue("visorbus_dev");
> -	if (!periodic_dev_workqueue) {
> -		POSTCODE_LINUX_2(CREATE_WORKQUEUE_PC, DIAG_SEVERITY_ERR);
> -		err = -ENOMEM;
> -		goto error;
> -	}
> -
>  	/* This enables us to receive notifications when devices appear for
>  	 * which this service partition is to be a server for.
>  	 */
> @@ -1297,10 +1277,6 @@ visorbus_exit(void)
>  	visorchipset_register_busdev(NULL, NULL, NULL);
>  	remove_all_visor_devices();
>  
> -	flush_workqueue(periodic_dev_workqueue); /* better not be any work! */
> -	destroy_workqueue(periodic_dev_workqueue);
> -	periodic_dev_workqueue = NULL;
> -
>  	list_for_each_safe(listentry, listtmp, &list_all_bus_instances) {
>  		struct visor_device *dev = list_entry(listentry,
>  						      struct visor_device,
> -- 
> 1.9.1
> 
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