Re: [PATCH] drm/i915: Always use kref tracking for all contexts.

[Mika Kuoppala <mika.kuoppala@linux.intel.com>]

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

> If we always initialize kref for the context, even if we are using fake
> contexts for hangstats when there is no hw support, we can forgo the
> dance to dereference the ctx->obj and inspect whether we are permitted
> to use kref inside i915_gem_context_reference() and _unreference().
>
> My ulterior motive here is to improve the debugging of a use-after-free
> of ctx->obj. This patch avoids the dereference here and instead forces
> the assertion checks associated with kref.
>
> v2: Refactor the fake contexts to being even more like the real
> contexts, so that there is much less duplicated and special case code.
>
> Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=76671
> Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
> Tested-by: lu hua <huax.lu@intel.com>
> Cc: Ben Widawsky <benjamin.widawsky@intel.com>
> ---
>  drivers/gpu/drm/i915/i915_drv.h            |   8 +-
>  drivers/gpu/drm/i915/i915_gem.c            |   2 +-
>  drivers/gpu/drm/i915/i915_gem_context.c    | 175 ++++++++++++-----------------
>  drivers/gpu/drm/i915/i915_gem_execbuffer.c |   2 +-
>  4 files changed, 76 insertions(+), 111 deletions(-)
>
> diff --git a/drivers/gpu/drm/i915/i915_drv.h b/drivers/gpu/drm/i915/i915_drv.h
> index c0e2de9d54b8..97bf793f6552 100644
> --- a/drivers/gpu/drm/i915/i915_drv.h
> +++ b/drivers/gpu/drm/i915/i915_drv.h
> @@ -2275,20 +2275,18 @@ int i915_gem_context_open(struct drm_device *dev, struct drm_file *file);
>  int i915_gem_context_enable(struct drm_i915_private *dev_priv);
>  void i915_gem_context_close(struct drm_device *dev, struct drm_file *file);
>  int i915_switch_context(struct intel_ring_buffer *ring,
> -			struct drm_file *file, struct i915_hw_context *to);
> +			struct i915_hw_context *to);
>  struct i915_hw_context *
>  i915_gem_context_get(struct drm_i915_file_private *file_priv, u32 id);
>  void i915_gem_context_free(struct kref *ctx_ref);
>  static inline void i915_gem_context_reference(struct i915_hw_context *ctx)
>  {
> -	if (ctx->obj && HAS_HW_CONTEXTS(ctx->obj->base.dev))
> -		kref_get(&ctx->ref);
> +	kref_get(&ctx->ref);
>  }
>  
>  static inline void i915_gem_context_unreference(struct i915_hw_context *ctx)
>  {
> -	if (ctx->obj && HAS_HW_CONTEXTS(ctx->obj->base.dev))
> -		kref_put(&ctx->ref, i915_gem_context_free);
> +	kref_put(&ctx->ref, i915_gem_context_free);
>  }
>  
>  static inline bool i915_gem_context_is_default(const struct i915_hw_context *c)
> diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
> index 221f6ab3a498..0ff65666c5f7 100644
> --- a/drivers/gpu/drm/i915/i915_gem.c
> +++ b/drivers/gpu/drm/i915/i915_gem.c
> @@ -2819,7 +2819,7 @@ int i915_gpu_idle(struct drm_device *dev)
>  
>  	/* Flush everything onto the inactive list. */
>  	for_each_ring(ring, dev_priv, i) {
> -		ret = i915_switch_context(ring, NULL, ring->default_context);
> +		ret = i915_switch_context(ring, ring->default_context);
>  		if (ret)
>  			return ret;
>  
> diff --git a/drivers/gpu/drm/i915/i915_gem_context.c b/drivers/gpu/drm/i915/i915_gem_context.c
> index edbad2787d65..1439cc6bd439 100644
> --- a/drivers/gpu/drm/i915/i915_gem_context.c
> +++ b/drivers/gpu/drm/i915/i915_gem_context.c
> @@ -96,9 +96,6 @@
>  #define GEN6_CONTEXT_ALIGN (64<<10)
>  #define GEN7_CONTEXT_ALIGN 4096
>  
> -static int do_switch(struct intel_ring_buffer *ring,
> -		     struct i915_hw_context *to);
> -
>  static void do_ppgtt_cleanup(struct i915_hw_ppgtt *ppgtt)
>  {
>  	struct drm_device *dev = ppgtt->base.dev;
> @@ -185,13 +182,15 @@ void i915_gem_context_free(struct kref *ctx_ref)
>  						   typeof(*ctx), ref);
>  	struct i915_hw_ppgtt *ppgtt = NULL;
>  
> -	/* We refcount even the aliasing PPGTT to keep the code symmetric */
> -	if (USES_PPGTT(ctx->obj->base.dev))
> -		ppgtt = ctx_to_ppgtt(ctx);
> +	if (ctx->obj) {
> +		/* We refcount even the aliasing PPGTT to keep the code symmetric */
> +		if (USES_PPGTT(ctx->obj->base.dev))
> +			ppgtt = ctx_to_ppgtt(ctx);
>  
> -	/* XXX: Free up the object before tearing down the address space, in
> -	 * case we're bound in the PPGTT */
> -	drm_gem_object_unreference(&ctx->obj->base);
> +		/* XXX: Free up the object before tearing down the address space, in
> +		 * case we're bound in the PPGTT */
> +		drm_gem_object_unreference(&ctx->obj->base);
> +	}
>  
>  	if (ppgtt)
>  		kref_put(&ppgtt->ref, ppgtt_release);
> @@ -232,32 +231,32 @@ __create_hw_context(struct drm_device *dev,
>  		return ERR_PTR(-ENOMEM);
>  
>  	kref_init(&ctx->ref);
> -	ctx->obj = i915_gem_alloc_object(dev, dev_priv->hw_context_size);
> -	INIT_LIST_HEAD(&ctx->link);
> -	if (ctx->obj == NULL) {
> -		kfree(ctx);
> -		DRM_DEBUG_DRIVER("Context object allocated failed\n");
> -		return ERR_PTR(-ENOMEM);
> -	}
> +	list_add_tail(&ctx->link, &dev_priv->context_list);
>  
> -	if (INTEL_INFO(dev)->gen >= 7) {
> -		ret = i915_gem_object_set_cache_level(ctx->obj,
> -						      I915_CACHE_L3_LLC);
> -		/* Failure shouldn't ever happen this early */
> -		if (WARN_ON(ret))
> +	if (dev_priv->hw_context_size) {
> +		ctx->obj = i915_gem_alloc_object(dev, dev_priv->hw_context_size);
> +		if (ctx->obj == NULL) {
> +			ret = -ENOMEM;
>  			goto err_out;
> -	}
> +		}
>  
> -	list_add_tail(&ctx->link, &dev_priv->context_list);
> +		if (INTEL_INFO(dev)->gen >= 7) {
> +			ret = i915_gem_object_set_cache_level(ctx->obj,
> +							      I915_CACHE_L3_LLC);
> +			/* Failure shouldn't ever happen this early */
> +			if (WARN_ON(ret))
> +				goto err_out;
> +		}
> +	}
>  
>  	/* Default context will never have a file_priv */
> -	if (file_priv == NULL)
> -		return ctx;
> -
> -	ret = idr_alloc(&file_priv->context_idr, ctx, DEFAULT_CONTEXT_ID, 0,
> -			GFP_KERNEL);
> -	if (ret < 0)
> -		goto err_out;
> +	if (file_priv != NULL) {
> +		ret = idr_alloc(&file_priv->context_idr, ctx,
> +				DEFAULT_CONTEXT_ID, 0, GFP_KERNEL);
> +		if (ret < 0)
> +			goto err_out;
> +	} else
> +		ret = DEFAULT_CONTEXT_ID;
>  
>  	ctx->file_priv = file_priv;
>  	ctx->id = ret;
> @@ -294,7 +293,7 @@ i915_gem_create_context(struct drm_device *dev,
>  	if (IS_ERR(ctx))
>  		return ctx;
>  
> -	if (is_global_default_ctx) {
> +	if (is_global_default_ctx && ctx->obj) {
>  		/* We may need to do things with the shrinker which
>  		 * require us to immediately switch back to the default
>  		 * context. This can cause a problem as pinning the
> @@ -342,7 +341,7 @@ i915_gem_create_context(struct drm_device *dev,
>  	return ctx;
>  
>  err_unpin:
> -	if (is_global_default_ctx)
> +	if (is_global_default_ctx && ctx->obj)
>  		i915_gem_object_ggtt_unpin(ctx->obj);
>  err_destroy:
>  	i915_gem_context_unreference(ctx);
> @@ -352,16 +351,14 @@ err_destroy:
>  void i915_gem_context_reset(struct drm_device *dev)
>  {
>  	struct drm_i915_private *dev_priv = dev->dev_private;
> -	struct intel_ring_buffer *ring;
>  	int i;
>  
> -	if (!HAS_HW_CONTEXTS(dev))
> -		return;
> -
>  	/* Prevent the hardware from restoring the last context (which hung) on
>  	 * the next switch */
>  	for (i = 0; i < I915_NUM_RINGS; i++) {
>  		struct i915_hw_context *dctx;
> +		struct intel_ring_buffer *ring;
> +
>  		if (!(INTEL_INFO(dev)->ring_mask & (1<<i)))
>  			continue;
>  
> @@ -377,7 +374,7 @@ void i915_gem_context_reset(struct drm_device *dev)
>  		if (ring->last_context == dctx)
>  			continue;
>  
> -		if (i == RCS) {
> +		if (dctx->obj && i == RCS) {
>  			WARN_ON(i915_gem_obj_ggtt_pin(dctx->obj,
>  						      get_context_alignment(dev), 0));
>  			/* Fake a finish/inactive */
> @@ -394,44 +391,35 @@ void i915_gem_context_reset(struct drm_device *dev)
>  int i915_gem_context_init(struct drm_device *dev)
>  {
>  	struct drm_i915_private *dev_priv = dev->dev_private;
> -	struct intel_ring_buffer *ring;
> +	struct i915_hw_context *ctx;
>  	int i;
>  
> -	if (!HAS_HW_CONTEXTS(dev))
> -		return 0;
> -
>  	/* Init should only be called once per module load. Eventually the
>  	 * restriction on the context_disabled check can be loosened. */
>  	if (WARN_ON(dev_priv->ring[RCS].default_context))
>  		return 0;
>  
> -	dev_priv->hw_context_size = round_up(get_context_size(dev), 4096);
> -
> -	if (dev_priv->hw_context_size > (1<<20)) {
> -		DRM_DEBUG_DRIVER("Disabling HW Contexts; invalid size\n");
> -		return -E2BIG;
> +	if (HAS_HW_CONTEXTS(dev)) {
> +		dev_priv->hw_context_size = round_up(get_context_size(dev), 4096);
> +		if (dev_priv->hw_context_size > (1<<20)) {
> +			DRM_DEBUG_DRIVER("Disabling HW Contexts; invalid size %d\n",
> +					 dev_priv->hw_context_size);
> +			dev_priv->hw_context_size = 0;
> +		}
>  	}
>  
> -	dev_priv->ring[RCS].default_context =
> -		i915_gem_create_context(dev, NULL, USES_PPGTT(dev));
> -
> -	if (IS_ERR_OR_NULL(dev_priv->ring[RCS].default_context)) {
> -		DRM_DEBUG_DRIVER("Disabling HW Contexts; create failed %ld\n",
> -				 PTR_ERR(dev_priv->ring[RCS].default_context));
> -		return PTR_ERR(dev_priv->ring[RCS].default_context);
> +	ctx = i915_gem_create_context(dev, NULL, USES_PPGTT(dev));
> +	if (IS_ERR(ctx)) {
> +		DRM_ERROR("Failed to create default global context (error %ld)\n",
> +			  PTR_ERR(ctx));
> +		return PTR_ERR(ctx);
>  	}
>  
> -	for (i = RCS + 1; i < I915_NUM_RINGS; i++) {
> -		if (!(INTEL_INFO(dev)->ring_mask & (1<<i)))
> -			continue;
> +	/* NB: RCS will hold a ref for all rings */
> +	for (i = 0; i < I915_NUM_RINGS; i++)
> +		dev_priv->ring[i].default_context = ctx;
>  
> -		ring = &dev_priv->ring[i];
> -
> -		/* NB: RCS will hold a ref for all rings */
> -		ring->default_context = dev_priv->ring[RCS].default_context;
> -	}
> -
> -	DRM_DEBUG_DRIVER("HW context support initialized\n");
> +	DRM_DEBUG_DRIVER("%s context support initialized\n", dev_priv->hw_context_size ? "HW" : "fake");
>  	return 0;
>  }
>  
> @@ -441,13 +429,11 @@ void i915_gem_context_fini(struct drm_device *dev)
>  	struct i915_hw_context *dctx = dev_priv->ring[RCS].default_context;
>  	int i;
>  
> -	if (!HAS_HW_CONTEXTS(dev))
> -		return;
> -
> -	/* The only known way to stop the gpu from accessing the hw context is
> -	 * to reset it. Do this as the very last operation to avoid confusing
> -	 * other code, leading to spurious errors. */
> -	intel_gpu_reset(dev);
> +	if (dev_priv->hw_context_size)
> +		/* The only known way to stop the gpu from accessing the hw context is
> +		 * to reset it. Do this as the very last operation to avoid confusing
> +		 * other code, leading to spurious errors. */
> +		intel_gpu_reset(dev);
>  
>  	/* When default context is created and switched to, base object refcount
>  	 * will be 2 (+1 from object creation and +1 from do_switch()).
> @@ -456,7 +442,7 @@ void i915_gem_context_fini(struct drm_device *dev)
>  	 * to offset the do_switch part, so that i915_gem_context_unreference()
>  	 * can then free the base object correctly. */
>  	WARN_ON(!dev_priv->ring[RCS].last_context);
> -	if (dev_priv->ring[RCS].last_context == dctx) {
> +	if (dctx->obj && dev_priv->ring[RCS].last_context == dctx) {
>  		/* Fake switch to NULL context */
>  		WARN_ON(dctx->obj->active);
>  		i915_gem_object_ggtt_unpin(dctx->obj);
> @@ -466,6 +452,7 @@ void i915_gem_context_fini(struct drm_device *dev)
>  
>  	for (i = 0; i < I915_NUM_RINGS; i++) {
>  		struct intel_ring_buffer *ring = &dev_priv->ring[i];
> +
>  		if (!(INTEL_INFO(dev)->ring_mask & (1<<i)))
>  			continue;
>  
> @@ -478,7 +465,6 @@ void i915_gem_context_fini(struct drm_device *dev)
>  
>  	i915_gem_object_ggtt_unpin(dctx->obj);
>  	i915_gem_context_unreference(dctx);
> -	dev_priv->mm.aliasing_ppgtt = NULL;
>  }
>  
>  int i915_gem_context_enable(struct drm_i915_private *dev_priv)
> @@ -486,9 +472,6 @@ int i915_gem_context_enable(struct drm_i915_private *dev_priv)
>  	struct intel_ring_buffer *ring;
>  	int ret, i;
>  
> -	if (!HAS_HW_CONTEXTS(dev_priv->dev))
> -		return 0;
> -
>  	/* This is the only place the aliasing PPGTT gets enabled, which means
>  	 * it has to happen before we bail on reset */
>  	if (dev_priv->mm.aliasing_ppgtt) {
> @@ -503,7 +486,7 @@ int i915_gem_context_enable(struct drm_i915_private *dev_priv)
>  	BUG_ON(!dev_priv->ring[RCS].default_context);
>  
>  	for_each_ring(ring, dev_priv, i) {
> -		ret = do_switch(ring, ring->default_context);
> +		ret = i915_switch_context(ring, ring->default_context);
>  		if (ret)
>  			return ret;
>  	}
> @@ -526,19 +509,6 @@ static int context_idr_cleanup(int id, void *p, void *data)
>  int i915_gem_context_open(struct drm_device *dev, struct drm_file *file)
>  {
>  	struct drm_i915_file_private *file_priv = file->driver_priv;
> -	struct drm_i915_private *dev_priv = dev->dev_private;
> -
> -	if (!HAS_HW_CONTEXTS(dev)) {
> -		/* Cheat for hang stats */
> -		file_priv->private_default_ctx =
> -			kzalloc(sizeof(struct i915_hw_context), GFP_KERNEL);
> -
> -		if (file_priv->private_default_ctx == NULL)
> -			return -ENOMEM;
> -
> -		file_priv->private_default_ctx->vm = &dev_priv->gtt.base;
> -		return 0;
> -	}
>  
>  	idr_init(&file_priv->context_idr);
>  
> @@ -559,14 +529,13 @@ void i915_gem_context_close(struct drm_device *dev, struct drm_file *file)
>  {
>  	struct drm_i915_file_private *file_priv = file->driver_priv;
>  
> -	if (!HAS_HW_CONTEXTS(dev)) {
> -		kfree(file_priv->private_default_ctx);
> +	if (IS_ERR_OR_NULL(file_priv->private_default_ctx))
>  		return;
> -	}
>  
>  	idr_for_each(&file_priv->context_idr, context_idr_cleanup, NULL);
> -	i915_gem_context_unreference(file_priv->private_default_ctx);
>  	idr_destroy(&file_priv->context_idr);
> +
> +	i915_gem_context_unreference(file_priv->private_default_ctx);
>  }
>  
>  struct i915_hw_context *
> @@ -574,9 +543,6 @@ i915_gem_context_get(struct drm_i915_file_private *file_priv, u32 id)
>  {
>  	struct i915_hw_context *ctx;
>  
> -	if (!HAS_HW_CONTEXTS(file_priv->dev_priv->dev))
> -		return file_priv->private_default_ctx;
> -
>  	ctx = (struct i915_hw_context *)idr_find(&file_priv->context_idr, id);
>  	if (!ctx)
>  		return ERR_PTR(-ENOENT);
> @@ -758,7 +724,6 @@ unpin_out:
>  /**
>   * i915_switch_context() - perform a GPU context switch.
>   * @ring: ring for which we'll execute the context switch
> - * @file_priv: file_priv associated with the context, may be NULL
>   * @to: the context to switch to
>   *
>   * The context life cycle is simple. The context refcount is incremented and
> @@ -767,18 +732,20 @@ unpin_out:
>   * object while letting the normal object tracking destroy the backing BO.
>   */
>  int i915_switch_context(struct intel_ring_buffer *ring,
> -			struct drm_file *file,
>  			struct i915_hw_context *to)
>  {
>  	struct drm_i915_private *dev_priv = ring->dev->dev_private;
>  
>  	WARN_ON(!mutex_is_locked(&dev_priv->dev->struct_mutex));
>  
> -	BUG_ON(file && to == NULL);
> -
> -	/* We have the fake context */
> -	if (!HAS_HW_CONTEXTS(ring->dev)) {
> -		ring->last_context = to;
> +	if (to->obj == NULL) { /* We have the fake context */
> +		if (to != ring->last_context) {
> +			if (to)
> +				i915_gem_context_reference(to);
                        ^^
Not needed as you have already oopsed earlier if it doesnt exist.

-Mika

> +			if (ring->last_context)
> +				i915_gem_context_unreference(ring->last_context);
> +			ring->last_context = to;
> +		}
>  		return 0;
>  	}
>  
> @@ -793,7 +760,7 @@ int i915_gem_context_create_ioctl(struct drm_device *dev, void *data,
>  	struct i915_hw_context *ctx;
>  	int ret;
>  
> -	if (!HAS_HW_CONTEXTS(dev))
> +	if (to_i915(dev)->hw_context_size == 0)
>  		return -ENODEV;
>  
>  	ret = i915_mutex_lock_interruptible(dev);
> diff --git a/drivers/gpu/drm/i915/i915_gem_execbuffer.c b/drivers/gpu/drm/i915/i915_gem_execbuffer.c
> index 7447160155a3..2c9d9cbaf653 100644
> --- a/drivers/gpu/drm/i915/i915_gem_execbuffer.c
> +++ b/drivers/gpu/drm/i915/i915_gem_execbuffer.c
> @@ -1221,7 +1221,7 @@ i915_gem_do_execbuffer(struct drm_device *dev, void *data,
>  	if (ret)
>  		goto err;
>  
> -	ret = i915_switch_context(ring, file, ctx);
> +	ret = i915_switch_context(ring, ctx);
>  	if (ret)
>  		goto err;
>  
> -- 
> 1.9.1
>
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