drm/i915: Always use kref tracking for all contexts.

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.

v3: Tweaks.
v4: Tweaks, minor.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=76671Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Tested-by: lu hua <huax.lu@intel.com>
Cc: Ben Widawsky <benjamin.widawsky@intel.com>
Cc: Mika Kuoppala <mika.kuoppala@intel.com>
Reviewed-by: Ben Widawsky <ben@bwidawsk.net>
[Jani: tiny change to backport to drm-intel-fixes.]
Signed-off-by: Jani Nikula <jani.nikula@intel.com>

drivers/gpu/drm/i915/i915_drv.h

@@ -2432,20 +2432,18 @@ 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)

drivers/gpu/drm/i915/i915_gem.c

@@ -2790,7 +2790,7 @@ 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;
 

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);
@@ -352,32 +351,22 @@ i915_gem_create_context(struct drm_device *dev,
 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;
-		if (!(INTEL_INFO(dev)->ring_mask & (1<<i)))
-			continue;
+		struct intel_ring_buffer *ring = &dev_priv->ring[i];
+		struct i915_hw_context *dctx = ring->default_context;
 
 		/* Do a fake switch to the default context */
-		ring = &dev_priv->ring[i];
-		dctx = ring->default_context;
-		if (WARN_ON(!dctx))
+		if (ring->last_context == dctx)
 			continue;
 
 		if (!ring->last_context)
 			continue;
 
-		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 +383,35 @@ 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;
-
-		ring = &dev_priv->ring[i];
+	/* NB: RCS will hold a ref for all rings */
+	for (i = 0; i < I915_NUM_RINGS; i++)
+		dev_priv->ring[i].default_context = ctx;
 
-		/* 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;
 }
 
@@ -441,33 +421,30 @@ 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);
-
-	/* When default context is created and switched to, base object refcount
-	 * will be 2 (+1 from object creation and +1 from do_switch()).
-	 * i915_gem_context_fini() will be called after gpu_idle() has switched
-	 * to default context. So we need to unreference the base object once
-	 * 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) {
-		/* Fake switch to NULL context */
-		WARN_ON(dctx->obj->active);
-		i915_gem_object_ggtt_unpin(dctx->obj);
-		i915_gem_context_unreference(dctx);
-		dev_priv->ring[RCS].last_context = NULL;
+	if (dctx->obj) {
+		/* 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()).
+		 * i915_gem_context_fini() will be called after gpu_idle() has switched
+		 * to default context. So we need to unreference the base object once
+		 * 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) {
+			/* Fake switch to NULL context */
+			WARN_ON(dctx->obj->active);
+			i915_gem_object_ggtt_unpin(dctx->obj);
+			i915_gem_context_unreference(dctx);
+			dev_priv->ring[RCS].last_context = NULL;
+		}
 	}
 
 	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;
 
 		if (ring->last_context)
 			i915_gem_context_unreference(ring->last_context);
@@ -478,7 +455,6 @@ 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 +462,6 @@ 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 +476,7 @@ 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 +499,6 @@ 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 +519,10 @@ 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);
-		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 +530,6 @@ 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 +711,6 @@ static int do_switch(struct intel_ring_buffer *ring,
 /**
  * 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
@@ -767,24 +719,30 @@ static int do_switch(struct intel_ring_buffer *ring,
  * 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) {
+			i915_gem_context_reference(to);
+			if (ring->last_context)
+				i915_gem_context_unreference(ring->last_context);
+			ring->last_context = to;
+		}
 		return 0;
 	}
 
 	return do_switch(ring, to);
 }
 
+static bool hw_context_enabled(struct drm_device *dev)
+{
+	return to_i915(dev)->hw_context_size;
+}
+
 int i915_gem_context_create_ioctl(struct drm_device *dev, void *data,
 				  struct drm_file *file)
 {
@@ -793,7 +751,7 @@ 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 (!hw_context_enabled(dev))
 		return -ENODEV;
 
 	ret = i915_mutex_lock_interruptible(dev);

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;