Re: [PATCH 1/4] drm/radeon: use RCU query for GEM_BUSY syscall

["Christian König" <deathsimple@vodafone.de>]

On 03.07.2015 01:54, Grigori Goronzy wrote:
> We don't need to call the (expensive) radeon_bo_wait, checking the
> fences via RCU is much faster. The reservation done by radeon_bo_wait
> does not save us from any race conditions.
>
> Signed-off-by: Grigori Goronzy <greg@chown.ath.cx>

Patche #1-#3 are Reviewed-by: Christian König <christian.koenig@amd.com>

Patch #4 is a nice catch, but as Michel already noted needs more 
thoughts for a complete fix.

Regards,
Christian.

> ---
>   drivers/gpu/drm/radeon/radeon_gem.c | 11 ++++++++---
>   1 file changed, 8 insertions(+), 3 deletions(-)
>
> diff --git a/drivers/gpu/drm/radeon/radeon_gem.c b/drivers/gpu/drm/radeon/radeon_gem.c
> index ac3c131..7199e19 100644
> --- a/drivers/gpu/drm/radeon/radeon_gem.c
> +++ b/drivers/gpu/drm/radeon/radeon_gem.c
> @@ -428,7 +428,6 @@ int radeon_gem_mmap_ioctl(struct drm_device *dev, void *data,
>   int radeon_gem_busy_ioctl(struct drm_device *dev, void *data,
>   			  struct drm_file *filp)
>   {
> -	struct radeon_device *rdev = dev->dev_private;
>   	struct drm_radeon_gem_busy *args = data;
>   	struct drm_gem_object *gobj;
>   	struct radeon_bo *robj;
> @@ -440,10 +439,16 @@ int radeon_gem_busy_ioctl(struct drm_device *dev, void *data,
>   		return -ENOENT;
>   	}
>   	robj = gem_to_radeon_bo(gobj);
> -	r = radeon_bo_wait(robj, &cur_placement, true);
> +
> +	r = reservation_object_test_signaled_rcu(robj->tbo.resv, true);
> +	if (r == 0)
> +		r = -EBUSY;
> +	else
> +		r = 0;
> +
> +	cur_placement = ACCESS_ONCE(robj->tbo.mem.mem_type);
>   	args->domain = radeon_mem_type_to_domain(cur_placement);
>   	drm_gem_object_unreference_unlocked(gobj);
> -	r = radeon_gem_handle_lockup(rdev, r);
>   	return r;
>   }
>   

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