Re: [PATCH] driver core / ACPI: Avoid device removal locking problems

[Gu Zheng]

Hi Rafael,

On 08/28/2013 05:45 AM, Rafael J. Wysocki wrote:

> On Tuesday, August 27, 2013 02:36:44 PM Tejun Heo wrote:
>> Hello,
>>
[...]
> 
> I've thought about that a bit over the last several hours and I'm still
> thinking that that patch is a bit overkill, because it will trigger the
> restart_syscall() for all cases when device_hotplug_lock is locked, even
> if they can't lead to any deadlocks.  The only deadlockish situation is
> when device *removal* is in progress when store_online(), for example,
> is called.
> 
> So to address that particular situation without adding too much overhead for
> other cases, I've come up with the appended patch (untested for now).
> 
> This is how it is supposed to work.
> 
> There are three "lock levels" for device hotplug, "normal", "remove"
> and "weak".  The difference is related to how __lock_device_hotplug()
> works.  Namely, if device hotplug is currently locked, that function
> will either block or return false, depending on the "current lock
> level" and its argument (the "new lock level").  The rules here are
> that false is returned immediately if the "current lock level" is
> "remove" and the "new lock level" is "weak".  The function blocks
> for all other combinations of the two.
> 
> There are two functions supposed to use device hotplug "lock levels"
> other than "normal": store_online() and acpi_scan_hot_remove().
> Everybody else is supposed to use "normal" (well, there are more
> potential users of "weak" in drivers/base/memory.c).
> 
> acpi_scan_hot_remove() uses the "remove" lock level to indicate
> that it is going to remove devices while holding device hotplug
> locked.  In turn, store_online() uses the "weak" lock level so
> that it doesn't block when devices are being removed with device
> hotplug locked, because that may lead to a deadlock.
> 
> show_online() actually doesn't need to lock device hotplug, but
> it is useful to serialize it with respect to device_offline()
> and device_online() (in case user space attempts to run them
> concurrently).

Yeah. I tested this one on latest kernel tree, it does make the splat go away.
Looking forward to the ACPI part one.:)

Regards,
Gu

> 
> ---
>  drivers/acpi/scan.c    |    4 +-
>  drivers/base/core.c    |   72 ++++++++++++++++++++++++++++++++++++++-----------
>  include/linux/device.h |   25 ++++++++++++++++-
>  3 files changed, 83 insertions(+), 18 deletions(-)
> 
> Index: linux-pm/drivers/base/core.c
> ===================================================================
> --- linux-pm.orig/drivers/base/core.c
> +++ linux-pm/drivers/base/core.c
> @@ -49,6 +49,55 @@ static struct kobject *dev_kobj;
>  struct kobject *sysfs_dev_char_kobj;
>  struct kobject *sysfs_dev_block_kobj;
>  
> +static struct {
> +	struct task_struct *holder;
> +	enum dev_hotplug_lock_type type;
> +	struct mutex lock; /* Synchronizes accesses to holder and type */
> +	wait_queue_head_t wait_queue;
> +} device_hotplug = {
> +	.holder = NULL,
> +	.type = DEV_HOTPLUG_LOCK_NONE,
> +	.lock = __MUTEX_INITIALIZER(device_hotplug.lock),
> +	.wait_queue = __WAIT_QUEUE_HEAD_INITIALIZER(device_hotplug.wait_queue),
> +};
> +
> +bool __lock_device_hotplug(enum dev_hotplug_lock_type type)
> +{
> +	DEFINE_WAIT(wait);
> +	bool ret = true;
> +
> +	mutex_lock(&device_hotplug.lock);
> +	for (;;) {
> +		prepare_to_wait(&device_hotplug.wait_queue, &wait,
> +				TASK_UNINTERRUPTIBLE);
> +		if (!device_hotplug.holder) {
> +			device_hotplug.holder = current;
> +			device_hotplug.type = type;
> +			break;
> +		} else if (type == DEV_HOTPLUG_LOCK_WEAK
> +		    && device_hotplug.type == DEV_HOTPLUG_LOCK_REMOVE) {
> +			ret = false;
> +			break;
> +		}
> +		mutex_unlock(&device_hotplug.lock);
> +		schedule();
> +		mutex_lock(&device_hotplug.lock);
> +	}
> +	finish_wait(&device_hotplug.wait_queue, &wait);
> +	mutex_unlock(&device_hotplug.lock);
> +	return ret;
> +}
> +
> +void unlock_device_hotplug(void)
> +{
> +	mutex_lock(&device_hotplug.lock);
> +	BUG_ON(device_hotplug.holder != current);
> +	device_hotplug.holder = NULL;
> +	device_hotplug.type = DEV_HOTPLUG_LOCK_NONE;
> +	wake_up(&device_hotplug.wait_queue);
> +	mutex_unlock(&device_hotplug.lock);
> +}
> +
>  #ifdef CONFIG_BLOCK
>  static inline int device_is_not_partition(struct device *dev)
>  {
> @@ -408,9 +457,10 @@ static ssize_t show_online(struct device
>  {
>  	bool val;
>  
> -	lock_device_hotplug();
> +	/* Serialize against device_online() and device_offline(). */
> +	device_lock(dev);
>  	val = !dev->offline;
> -	unlock_device_hotplug();
> +	device_unlock(dev);
>  	return sprintf(buf, "%u\n", val);
>  }
>  
> @@ -424,7 +474,11 @@ static ssize_t store_online(struct devic
>  	if (ret < 0)
>  		return ret;
>  
> -	lock_device_hotplug();
> +	if (!__lock_device_hotplug(DEV_HOTPLUG_LOCK_WEAK)) {
> +		/* Avoid busy looping (10 ms delay should do). */
> +		msleep(10);
> +		return restart_syscall();
> +	}
>  	ret = val ? device_online(dev) : device_offline(dev);
>  	unlock_device_hotplug();
>  	return ret < 0 ? ret : count;
> @@ -1479,18 +1533,6 @@ EXPORT_SYMBOL_GPL(put_device);
>  EXPORT_SYMBOL_GPL(device_create_file);
>  EXPORT_SYMBOL_GPL(device_remove_file);
>  
> -static DEFINE_MUTEX(device_hotplug_lock);
> -
> -void lock_device_hotplug(void)
> -{
> -	mutex_lock(&device_hotplug_lock);
> -}
> -
> -void unlock_device_hotplug(void)
> -{
> -	mutex_unlock(&device_hotplug_lock);
> -}
> -
>  static int device_check_offline(struct device *dev, void *not_used)
>  {
>  	int ret;
> Index: linux-pm/include/linux/device.h
> ===================================================================
> --- linux-pm.orig/include/linux/device.h
> +++ linux-pm/include/linux/device.h
> @@ -893,8 +893,31 @@ static inline bool device_supports_offli
>  	return dev->bus && dev->bus->offline && dev->bus->online;
>  }
>  
> -extern void lock_device_hotplug(void);
> +enum dev_hotplug_lock_type {
> +	DEV_HOTPLUG_LOCK_NONE,
> +	DEV_HOTPLUG_LOCK_NORMAL,
> +	DEV_HOTPLUG_LOCK_REMOVE,
> +	DEV_HOTPLUG_LOCK_WEAK,
> +};
> +
> +extern bool __lock_device_hotplug(enum dev_hotplug_lock_type);
>  extern void unlock_device_hotplug(void);
> +
> +static inline void lock_device_hotplug(void)
> +{
> +	__lock_device_hotplug(DEV_HOTPLUG_LOCK_NORMAL);
> +}
> +
> +static inline void lock_device_hot_remove(void)
> +{
> +	__lock_device_hotplug(DEV_HOTPLUG_LOCK_REMOVE);
> +}
> +
> +static inline void unlock_device_hot_remove(void)
> +{
> +	unlock_device_hotplug();
> +}
> +
>  extern int device_offline(struct device *dev);
>  extern int device_online(struct device *dev);
>  /*
> Index: linux-pm/drivers/acpi/scan.c
> ===================================================================
> --- linux-pm.orig/drivers/acpi/scan.c
> +++ linux-pm/drivers/acpi/scan.c
> @@ -204,7 +204,7 @@ static int acpi_scan_hot_remove(struct a
>  		return -EINVAL;
>  	}
>  
> -	lock_device_hotplug();
> +	lock_device_hot_remove();
>  
>  	/*
>  	 * Carry out two passes here and ignore errors in the first pass,
> @@ -249,7 +249,7 @@ static int acpi_scan_hot_remove(struct a
>  
>  	acpi_bus_trim(device);
>  
> -	unlock_device_hotplug();
> +	unlock_device_hot_remove();
>  
>  	/* Device node has been unregistered. */
>  	put_device(&device->dev);
> 
> 


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