Re: [PATCH 2/2] drm/i915: Prevent concurrent tiling/framebuffer modifications

["Ville Syrjälä" <ville.syrjala@linux.intel.com>]

On Tue, Feb 28, 2017 at 04:22:33PM +0000, Chris Wilson wrote:
> Reintroduce a lock around tiling vs framebuffer creation to prevent
> modification of the obj->tiling_and_stride whilst the framebuffer is
> being created. Rather than use struct_mutex once again, use the
> per-object lock - this will also be required in future to prevent
> changing the tiling whilst submitting rendering.
> 
> Fixes: 24dbf51a5517 ("drm/i915: struct_mutex is not required for allocating the framebuffer")
> Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
> Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
> ---
>  drivers/gpu/drm/i915/i915_gem_object.h   | 18 +++++++++++++++++-
>  drivers/gpu/drm/i915/i915_gem_shrinker.c |  2 +-
>  drivers/gpu/drm/i915/i915_gem_tiling.c   |  9 ++++++++-
>  drivers/gpu/drm/i915/intel_display.c     | 25 ++++++++++++++++---------
>  4 files changed, 42 insertions(+), 12 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/i915_gem_object.h b/drivers/gpu/drm/i915/i915_gem_object.h
> index ad1bc0b1a0c2..8c02c8ec2a3b 100644
> --- a/drivers/gpu/drm/i915/i915_gem_object.h
> +++ b/drivers/gpu/drm/i915/i915_gem_object.h
> @@ -169,7 +169,7 @@ struct drm_i915_gem_object {
>  	struct reservation_object *resv;
>  
>  	/** References from framebuffers, locks out tiling changes. */
> -	atomic_t framebuffer_references;
> +	unsigned int framebuffer_references;
>  
>  	/** Record of address bit 17 of each page at last unbind. */
>  	unsigned long *bit_17;
> @@ -263,6 +263,16 @@ extern void drm_gem_object_unreference(struct drm_gem_object *);
>  __deprecated
>  extern void drm_gem_object_unreference_unlocked(struct drm_gem_object *);
>  
> +static inline void i915_gem_object_lock(struct drm_i915_gem_object *obj)
> +{
> +	reservation_object_lock(obj->resv, NULL);
> +}
> +
> +static inline void i915_gem_object_unlock(struct drm_i915_gem_object *obj)
> +{
> +	reservation_object_unlock(obj->resv);
> +}
> +
>  static inline bool
>  i915_gem_object_has_struct_page(const struct drm_i915_gem_object *obj)
>  {
> @@ -303,6 +313,12 @@ i915_gem_object_clear_active_reference(struct drm_i915_gem_object *obj)
>  
>  void __i915_gem_object_release_unless_active(struct drm_i915_gem_object *obj);
>  
> +static inline bool
> +i915_gem_object_is_framebuffer(const struct drm_i915_gem_object *obj)
> +{
> +	return READ_ONCE(obj->framebuffer_references);
> +}
> +
>  static inline unsigned int
>  i915_gem_object_get_tiling(struct drm_i915_gem_object *obj)
>  {
> diff --git a/drivers/gpu/drm/i915/i915_gem_shrinker.c b/drivers/gpu/drm/i915/i915_gem_shrinker.c
> index 7e3bb48e043e..630697001b38 100644
> --- a/drivers/gpu/drm/i915/i915_gem_shrinker.c
> +++ b/drivers/gpu/drm/i915/i915_gem_shrinker.c
> @@ -210,7 +210,7 @@ i915_gem_shrink(struct drm_i915_private *dev_priv,
>  
>  			if (!(flags & I915_SHRINK_ACTIVE) &&
>  			    (i915_gem_object_is_active(obj) ||
> -			     atomic_read(&obj->framebuffer_references)))
> +			     i915_gem_object_is_framebuffer(obj)))
>  				continue;
>  
>  			if (!can_release_pages(obj))
> diff --git a/drivers/gpu/drm/i915/i915_gem_tiling.c b/drivers/gpu/drm/i915/i915_gem_tiling.c
> index c1d669e32f41..ad5e05f6b836 100644
> --- a/drivers/gpu/drm/i915/i915_gem_tiling.c
> +++ b/drivers/gpu/drm/i915/i915_gem_tiling.c
> @@ -238,7 +238,7 @@ i915_gem_object_set_tiling(struct drm_i915_gem_object *obj,
>  	if ((tiling | stride) == obj->tiling_and_stride)
>  		return 0;
>  
> -	if (atomic_read(&obj->framebuffer_references))
> +	if (i915_gem_object_is_framebuffer(obj))
>  		return -EBUSY;
>  
>  	/* We need to rebind the object if its current allocation
> @@ -258,6 +258,12 @@ i915_gem_object_set_tiling(struct drm_i915_gem_object *obj,
>  	if (err)
>  		return err;
>  
> +	i915_gem_object_lock(obj);
> +	if (i915_gem_object_is_framebuffer(obj)) {
> +		i915_gem_object_unlock(obj);
> +		return -EBUSY;
> +	}
> +
>  	/* If the memory has unknown (i.e. varying) swizzling, we pin the
>  	 * pages to prevent them being swapped out and causing corruption
>  	 * due to the change in swizzling.
> @@ -294,6 +300,7 @@ i915_gem_object_set_tiling(struct drm_i915_gem_object *obj,
>  	}
>  
>  	obj->tiling_and_stride = tiling | stride;
> +	i915_gem_object_unlock(obj);
>  
>  	/* Force the fence to be reacquired for GTT access */
>  	i915_gem_release_mmap(obj);
> diff --git a/drivers/gpu/drm/i915/intel_display.c b/drivers/gpu/drm/i915/intel_display.c
> index 77936ddd860a..62a1e628e399 100644
> --- a/drivers/gpu/drm/i915/intel_display.c
> +++ b/drivers/gpu/drm/i915/intel_display.c
> @@ -14255,7 +14255,10 @@ static void intel_user_framebuffer_destroy(struct drm_framebuffer *fb)
>  
>  	drm_framebuffer_cleanup(fb);
>  
> -	WARN_ON(atomic_dec_return(&intel_fb->obj->framebuffer_references) < 0);
> +	i915_gem_object_lock(intel_fb->obj);
> +	WARN_ON(!intel_fb->obj->framebuffer_references--);
> +	i915_gem_object_unlock(intel_fb->obj);
> +
>  	i915_gem_object_put(intel_fb->obj);
>  
>  	kfree(intel_fb);
> @@ -14332,12 +14335,16 @@ static int intel_framebuffer_init(struct intel_framebuffer *intel_fb,
>  				  struct drm_mode_fb_cmd2 *mode_cmd)
>  {
>  	struct drm_i915_private *dev_priv = to_i915(obj->base.dev);
> -	unsigned int tiling = i915_gem_object_get_tiling(obj);
> -	u32 pitch_limit, stride_alignment;
>  	struct drm_format_name_buf format_name;
> +	u32 pitch_limit, stride_alignment;
> +	unsigned int tiling, stride;
>  	int ret = -EINVAL;
>  
> -	atomic_inc(&obj->framebuffer_references);
> +	i915_gem_object_lock(obj);
> +	obj->framebuffer_references++;
> +	tiling = i915_gem_object_get_tiling(obj);
> +	stride = i915_gem_object_get_stride(obj);
> +	i915_gem_object_unlock(obj);

I can't say I'm really up to date on the object locking stuff, but
from the display POV this looks all right to me. So good enough for me
:)

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>

>  
>  	if (mode_cmd->flags & DRM_MODE_FB_MODIFIERS) {
>  		/*
> @@ -14409,11 +14416,9 @@ static int intel_framebuffer_init(struct intel_framebuffer *intel_fb,
>  	 * If there's a fence, enforce that
>  	 * the fb pitch and fence stride match.
>  	 */
> -	if (tiling != I915_TILING_NONE &&
> -	    mode_cmd->pitches[0] != i915_gem_object_get_stride(obj)) {
> +	if (tiling != I915_TILING_NONE && mode_cmd->pitches[0] !=  stride) {
>  		DRM_DEBUG("pitch (%d) must match tiling stride (%d)\n",
> -			  mode_cmd->pitches[0],
> -			  i915_gem_object_get_stride(obj));
> +			  mode_cmd->pitches[0], stride);
>  		goto err;
>  	}
>  
> @@ -14494,7 +14499,9 @@ static int intel_framebuffer_init(struct intel_framebuffer *intel_fb,
>  	return 0;
>  
>  err:
> -	atomic_dec(&obj->framebuffer_references);
> +	i915_gem_object_lock(obj);
> +	obj->framebuffer_references--;
> +	i915_gem_object_unlock(obj);
>  	return ret;
>  }
>  
> -- 
> 2.11.0

-- 
Ville Syrjälä
Intel OTC
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