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Subject: Re: [PATCH] drm: Use srcu to protect drm_device.unplugged
To:     linux-kernel@vger.kernel.org, dri-devel@lists.freedesktop.org,
        airlied@linux.ie, daniel.vetter@intel.com, seanpaul@chromium.org,
        gustavo@padovan.org, intel-gfx@lists.freedesktop.org,
        Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>
References: <1522222715-11814-1-git-send-email-andr2000@gmail.com>
 <20180328150926.GC3881@phenom.ffwll.local>
From:   Oleksandr Andrushchenko <andr2000@gmail.com>
Message-ID: <804f94d8-9f21-159b-220e-ca420a83c176@gmail.com>
Date:   Thu, 29 Mar 2018 14:18:44 +0300
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On 03/28/2018 06:09 PM, Daniel Vetter wrote:
> On Wed, Mar 28, 2018 at 10:38:35AM +0300, Oleksandr Andrushchenko wrote:
>> From: Noralf Trønnes <noralf@tronnes.org>
>>
>> Use srcu to protect drm_device.unplugged in a race free manner.
>> Drivers can use drm_dev_enter()/drm_dev_exit() to protect and mark
>> sections preventing access to device resources that are not available
>> after the device is gone.
>>
>> Suggested-by: Daniel Vetter <daniel.vetter@ffwll.ch>
>> Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
>> Signed-off-by: Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>
>> Reviewed-by: Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>
>> Tested-by: Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>
>> Cc: intel-gfx@lists.freedesktop.org
> Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
>
> Oleksandr, please push to drm-misc-next once you have dim tools setup up
> and everything.
Pushed to drm-misc-next
>
> Thanks, Daniel
Thank you,
Oleksandr
>> ---
>>   drivers/gpu/drm/drm_drv.c | 54 ++++++++++++++++++++++++++++++++++++++++++-----
>>   include/drm/drm_device.h  |  9 +++++++-
>>   include/drm/drm_drv.h     | 15 +++++++++----
>>   3 files changed, 68 insertions(+), 10 deletions(-)
>>
>> diff --git a/drivers/gpu/drm/drm_drv.c b/drivers/gpu/drm/drm_drv.c
>> index a1b9338736e3..32a83b41ab61 100644
>> --- a/drivers/gpu/drm/drm_drv.c
>> +++ b/drivers/gpu/drm/drm_drv.c
>> @@ -32,6 +32,7 @@
>>   #include <linux/moduleparam.h>
>>   #include <linux/mount.h>
>>   #include <linux/slab.h>
>> +#include <linux/srcu.h>
>>   
>>   #include <drm/drm_drv.h>
>>   #include <drm/drmP.h>
>> @@ -75,6 +76,8 @@ static bool drm_core_init_complete = false;
>>   
>>   static struct dentry *drm_debugfs_root;
>>   
>> +DEFINE_STATIC_SRCU(drm_unplug_srcu);
>> +
>>   /*
>>    * DRM Minors
>>    * A DRM device can provide several char-dev interfaces on the DRM-Major. Each
>> @@ -318,18 +321,51 @@ void drm_put_dev(struct drm_device *dev)
>>   }
>>   EXPORT_SYMBOL(drm_put_dev);
>>   
>> -static void drm_device_set_unplugged(struct drm_device *dev)
>> +/**
>> + * drm_dev_enter - Enter device critical section
>> + * @dev: DRM device
>> + * @idx: Pointer to index that will be passed to the matching drm_dev_exit()
>> + *
>> + * This function marks and protects the beginning of a section that should not
>> + * be entered after the device has been unplugged. The section end is marked
>> + * with drm_dev_exit(). Calls to this function can be nested.
>> + *
>> + * Returns:
>> + * True if it is OK to enter the section, false otherwise.
>> + */
>> +bool drm_dev_enter(struct drm_device *dev, int *idx)
>> +{
>> +	*idx = srcu_read_lock(&drm_unplug_srcu);
>> +
>> +	if (dev->unplugged) {
>> +		srcu_read_unlock(&drm_unplug_srcu, *idx);
>> +		return false;
>> +	}
>> +
>> +	return true;
>> +}
>> +EXPORT_SYMBOL(drm_dev_enter);
>> +
>> +/**
>> + * drm_dev_exit - Exit device critical section
>> + * @idx: index returned from drm_dev_enter()
>> + *
>> + * This function marks the end of a section that should not be entered after
>> + * the device has been unplugged.
>> + */
>> +void drm_dev_exit(int idx)
>>   {
>> -	smp_wmb();
>> -	atomic_set(&dev->unplugged, 1);
>> +	srcu_read_unlock(&drm_unplug_srcu, idx);
>>   }
>> +EXPORT_SYMBOL(drm_dev_exit);
>>   
>>   /**
>>    * drm_dev_unplug - unplug a DRM device
>>    * @dev: DRM device
>>    *
>>    * This unplugs a hotpluggable DRM device, which makes it inaccessible to
>> - * userspace operations. Entry-points can use drm_dev_is_unplugged(). This
>> + * userspace operations. Entry-points can use drm_dev_enter() and
>> + * drm_dev_exit() to protect device resources in a race free manner. This
>>    * essentially unregisters the device like drm_dev_unregister(), but can be
>>    * called while there are still open users of @dev.
>>    */
>> @@ -338,10 +374,18 @@ void drm_dev_unplug(struct drm_device *dev)
>>   	drm_dev_unregister(dev);
>>   
>>   	mutex_lock(&drm_global_mutex);
>> -	drm_device_set_unplugged(dev);
>>   	if (dev->open_count == 0)
>>   		drm_dev_put(dev);
>>   	mutex_unlock(&drm_global_mutex);
>> +
>> +	/*
>> +	 * After synchronizing any critical read section is guaranteed to see
>> +	 * the new value of ->unplugged, and any critical section which might
>> +	 * still have seen the old value of ->unplugged is guaranteed to have
>> +	 * finished.
>> +	 */
>> +	dev->unplugged = true;
>> +	synchronize_srcu(&drm_unplug_srcu);
>>   }
>>   EXPORT_SYMBOL(drm_dev_unplug);
>>   
>> diff --git a/include/drm/drm_device.h b/include/drm/drm_device.h
>> index 7c4fa32f3fc6..3a0eac2885b7 100644
>> --- a/include/drm/drm_device.h
>> +++ b/include/drm/drm_device.h
>> @@ -46,7 +46,14 @@ struct drm_device {
>>   	/* currently active master for this device. Protected by master_mutex */
>>   	struct drm_master *master;
>>   
>> -	atomic_t unplugged;			/**< Flag whether dev is dead */
>> +	/**
>> +	 * @unplugged:
>> +	 *
>> +	 * Flag to tell if the device has been unplugged.
>> +	 * See drm_dev_enter() and drm_dev_is_unplugged().
>> +	 */
>> +	bool unplugged;
>> +
>>   	struct inode *anon_inode;		/**< inode for private address-space */
>>   	char *unique;				/**< unique name of the device */
>>   	/*@} */
>> diff --git a/include/drm/drm_drv.h b/include/drm/drm_drv.h
>> index d23dcdd1bd95..7e545f5f94d3 100644
>> --- a/include/drm/drm_drv.h
>> +++ b/include/drm/drm_drv.h
>> @@ -624,6 +624,8 @@ void drm_dev_get(struct drm_device *dev);
>>   void drm_dev_put(struct drm_device *dev);
>>   void drm_dev_unref(struct drm_device *dev);
>>   void drm_put_dev(struct drm_device *dev);
>> +bool drm_dev_enter(struct drm_device *dev, int *idx);
>> +void drm_dev_exit(int idx);
>>   void drm_dev_unplug(struct drm_device *dev);
>>   
>>   /**
>> @@ -635,11 +637,16 @@ void drm_dev_unplug(struct drm_device *dev);
>>    * unplugged, these two functions guarantee that any store before calling
>>    * drm_dev_unplug() is visible to callers of this function after it completes
>>    */
>> -static inline int drm_dev_is_unplugged(struct drm_device *dev)
>> +static inline bool drm_dev_is_unplugged(struct drm_device *dev)
>>   {
>> -	int ret = atomic_read(&dev->unplugged);
>> -	smp_rmb();
>> -	return ret;
>> +	int idx;
>> +
>> +	if (drm_dev_enter(dev, &idx)) {
>> +		drm_dev_exit(idx);
>> +		return false;
>> +	}
>> +
>> +	return true;
>>   }
>>   
>>   
>> -- 
>> 2.7.4
>>
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