[PATCH] drm/i915: Remove locking for get-caching query

[PATCH] drm/i915: Remove locking for get-caching query

[Chris Wilson]

Reading a single value from the object, the locking only provides futile
protection against userspace races. The locking is useless so remove it.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
---
 drivers/gpu/drm/i915/i915_gem.c | 17 ++++-------------
 1 file changed, 4 insertions(+), 13 deletions(-)

diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
index 2b2b74dbb446..d071d0af2a6c 100644
--- a/drivers/gpu/drm/i915/i915_gem.c
+++ b/drivers/gpu/drm/i915/i915_gem.c
@@ -3983,17 +3983,10 @@ int i915_gem_get_caching_ioctl(struct drm_device *dev, void *data,
 {
 	struct drm_i915_gem_caching *args = data;
 	struct drm_i915_gem_object *obj;
-	int ret;
-
-	ret = i915_mutex_lock_interruptible(dev);
-	if (ret)
-		return ret;
 
 	obj = to_intel_bo(drm_gem_object_lookup(dev, file, args->handle));
-	if (&obj->base == NULL) {
-		ret = -ENOENT;
-		goto unlock;
-	}
+	if (&obj->base == NULL)
+		return -ENOENT;
 
 	switch (obj->cache_level) {
 	case I915_CACHE_LLC:
@@ -4010,10 +4003,8 @@ int i915_gem_get_caching_ioctl(struct drm_device *dev, void *data,
 		break;
 	}
 
-	drm_gem_object_unreference(&obj->base);
-unlock:
-	mutex_unlock(&dev->struct_mutex);
-	return ret;
+	drm_gem_object_unreference_unlocked(&obj->base);
+	return 0;
 }
 
 int i915_gem_set_caching_ioctl(struct drm_device *dev, void *data,
-- 
2.1.4

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Re: [PATCH] drm/i915: Remove locking for get-caching query

[Daniel Vetter]

On Thu, May 07, 2015 at 12:14:55PM +0100, Chris Wilson wrote:
> Reading a single value from the object, the locking only provides futile
> protection against userspace races. The locking is useless so remove it.
> 
> Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
> ---
>  drivers/gpu/drm/i915/i915_gem.c | 17 ++++-------------
>  1 file changed, 4 insertions(+), 13 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
> index 2b2b74dbb446..d071d0af2a6c 100644
> --- a/drivers/gpu/drm/i915/i915_gem.c
> +++ b/drivers/gpu/drm/i915/i915_gem.c
> @@ -3983,17 +3983,10 @@ int i915_gem_get_caching_ioctl(struct drm_device *dev, void *data,
>  {
>  	struct drm_i915_gem_caching *args = data;
>  	struct drm_i915_gem_object *obj;
> -	int ret;
> -
> -	ret = i915_mutex_lock_interruptible(dev);
> -	if (ret)
> -		return ret;
>  
>  	obj = to_intel_bo(drm_gem_object_lookup(dev, file, args->handle));
> -	if (&obj->base == NULL) {
> -		ret = -ENOENT;
> -		goto unlock;
> -	}
> +	if (&obj->base == NULL)
> +		return -ENOENT;
>  
>  	switch (obj->cache_level) {

Wrap this in ACCESS_ONCE, just for documentation purpose? Can do while
applying if you ack.
-Daniel

>  	case I915_CACHE_LLC:
> @@ -4010,10 +4003,8 @@ int i915_gem_get_caching_ioctl(struct drm_device *dev, void *data,
>  		break;
>  	}
>  
> -	drm_gem_object_unreference(&obj->base);
> -unlock:
> -	mutex_unlock(&dev->struct_mutex);
> -	return ret;
> +	drm_gem_object_unreference_unlocked(&obj->base);
> +	return 0;
>  }
>  
>  int i915_gem_set_caching_ioctl(struct drm_device *dev, void *data,
> -- 
> 2.1.4
> 
> _______________________________________________
> Intel-gfx mailing list
> Intel-gfx@lists.freedesktop.org
> http://lists.freedesktop.org/mailman/listinfo/intel-gfx

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch
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Re: [PATCH] drm/i915: Remove locking for get-caching query

[Chris Wilson]

On Thu, May 07, 2015 at 03:22:41PM +0200, Daniel Vetter wrote:
> On Thu, May 07, 2015 at 12:14:55PM +0100, Chris Wilson wrote:
> > Reading a single value from the object, the locking only provides futile
> > protection against userspace races. The locking is useless so remove it.
> > 
> > Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
> > ---
> >  drivers/gpu/drm/i915/i915_gem.c | 17 ++++-------------
> >  1 file changed, 4 insertions(+), 13 deletions(-)
> > 
> > diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
> > index 2b2b74dbb446..d071d0af2a6c 100644
> > --- a/drivers/gpu/drm/i915/i915_gem.c
> > +++ b/drivers/gpu/drm/i915/i915_gem.c
> > @@ -3983,17 +3983,10 @@ int i915_gem_get_caching_ioctl(struct drm_device *dev, void *data,
> >  {
> >  	struct drm_i915_gem_caching *args = data;
> >  	struct drm_i915_gem_object *obj;
> > -	int ret;
> > -
> > -	ret = i915_mutex_lock_interruptible(dev);
> > -	if (ret)
> > -		return ret;
> >  
> >  	obj = to_intel_bo(drm_gem_object_lookup(dev, file, args->handle));
> > -	if (&obj->base == NULL) {
> > -		ret = -ENOENT;
> > -		goto unlock;
> > -	}
> > +	if (&obj->base == NULL)
> > +		return -ENOENT;
> >  
> >  	switch (obj->cache_level) {
> 
> Wrap this in ACCESS_ONCE, just for documentation purpose? Can do while
> applying if you ack.

I was going to but I thought obj->cache_level was another bitfield... It
is :|
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre
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Re: [PATCH] drm/i915: Remove locking for get-caching query

[Mika Kuoppala]

Chris Wilson <chris@chris-wilson.co.uk> writes:

> Reading a single value from the object, the locking only provides futile
> protection against userspace races. The locking is useless so remove it.
>
> Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>

Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>

> ---
>  drivers/gpu/drm/i915/i915_gem.c | 17 ++++-------------
>  1 file changed, 4 insertions(+), 13 deletions(-)
>
> diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
> index 2b2b74dbb446..d071d0af2a6c 100644
> --- a/drivers/gpu/drm/i915/i915_gem.c
> +++ b/drivers/gpu/drm/i915/i915_gem.c
> @@ -3983,17 +3983,10 @@ int i915_gem_get_caching_ioctl(struct drm_device *dev, void *data,
>  {
>  	struct drm_i915_gem_caching *args = data;
>  	struct drm_i915_gem_object *obj;
> -	int ret;
> -
> -	ret = i915_mutex_lock_interruptible(dev);
> -	if (ret)
> -		return ret;
>  
>  	obj = to_intel_bo(drm_gem_object_lookup(dev, file, args->handle));
> -	if (&obj->base == NULL) {
> -		ret = -ENOENT;
> -		goto unlock;
> -	}
> +	if (&obj->base == NULL)
> +		return -ENOENT;
>  
>  	switch (obj->cache_level) {
>  	case I915_CACHE_LLC:
> @@ -4010,10 +4003,8 @@ int i915_gem_get_caching_ioctl(struct drm_device *dev, void *data,
>  		break;
>  	}
>  
> -	drm_gem_object_unreference(&obj->base);
> -unlock:
> -	mutex_unlock(&dev->struct_mutex);
> -	return ret;
> +	drm_gem_object_unreference_unlocked(&obj->base);
> +	return 0;
>  }
>  
>  int i915_gem_set_caching_ioctl(struct drm_device *dev, void *data,
> -- 
> 2.1.4
>
> _______________________________________________
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> Intel-gfx@lists.freedesktop.org
> http://lists.freedesktop.org/mailman/listinfo/intel-gfx
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Re: [PATCH] drm/i915: Remove locking for get-caching query

[shuang.he]

Tested-By: Intel Graphics QA PRTS (Patch Regression Test System Contact: shuang.he@intel.com)
Task id: 6343
-------------------------------------Summary-------------------------------------
Platform          Delta          drm-intel-nightly          Series Applied
PNV                 -1              199/199              198/199
ILK                                  302/302              302/302
SNB                                  316/316              316/316
IVB                                  342/342              342/342
BYT                                  286/286              286/286
BDW                                  321/321              321/321
-------------------------------------Detailed-------------------------------------
Platform  Test                                drm-intel-nightly          Series Applied
*PNV  igt@gem_userptr_blits@forked-sync-swapping-interruptible      PASS(1)      INIT(1)
Note: You need to pay more attention to line start with '*'
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