drm/i915: Enable rcu-only context lookups

Whilst the contents of the context is still protected by the big
struct_mutex, this is not much of an improvement. It is just one tiny
step towards reducing our BKL.
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170620110547.15947-3-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/i915_drv.h

@@ -3533,16 +3533,22 @@ void i915_gem_object_do_bit_17_swizzle(struct drm_i915_gem_object *obj,
 void i915_gem_object_save_bit_17_swizzle(struct drm_i915_gem_object *obj,
 					 struct sg_table *pages);
 
+static inline struct i915_gem_context *
+__i915_gem_context_lookup_rcu(struct drm_i915_file_private *file_priv, u32 id)
+{
+	return idr_find(&file_priv->context_idr, id);
+}
+
 static inline struct i915_gem_context *
 i915_gem_context_lookup(struct drm_i915_file_private *file_priv, u32 id)
 {
 	struct i915_gem_context *ctx;
 
-	lockdep_assert_held(&file_priv->dev_priv->drm.struct_mutex);
-
-	ctx = idr_find(&file_priv->context_idr, id);
-	if (!ctx)
-		return ERR_PTR(-ENOENT);
+	rcu_read_lock();
+	ctx = __i915_gem_context_lookup_rcu(file_priv, id);
+	if (ctx && !kref_get_unless_zero(&ctx->ref))
+		ctx = NULL;
+	rcu_read_unlock();
 
 	return ctx;
 }

drivers/gpu/drm/i915/i915_gem_context.c

@@ -187,7 +187,7 @@ static void i915_gem_context_free(struct i915_gem_context *ctx)
 	list_del(&ctx->link);
 
 	ida_simple_remove(&ctx->i915->contexts.hw_ida, ctx->hw_id);
-	kfree(ctx);
+	kfree_rcu(ctx, rcu);
 }
 
 static void contexts_free(struct drm_i915_private *i915)
@@ -1021,20 +1021,19 @@ int i915_gem_context_destroy_ioctl(struct drm_device *dev, void *data,
 	if (args->ctx_id == DEFAULT_CONTEXT_HANDLE)
 		return -ENOENT;
 
-	ret = i915_mutex_lock_interruptible(dev);
-	if (ret)
-		return ret;
-
 	ctx = i915_gem_context_lookup(file_priv, args->ctx_id);
-	if (IS_ERR(ctx)) {
-		mutex_unlock(&dev->struct_mutex);
-		return PTR_ERR(ctx);
-	}
+	if (!ctx)
+		return -ENOENT;
+
+	ret = mutex_lock_interruptible(&dev->struct_mutex);
+	if (ret)
+		goto out;
 
 	__destroy_hw_context(ctx, file_priv);
 	mutex_unlock(&dev->struct_mutex);
 
-	DRM_DEBUG("HW context %d destroyed\n", args->ctx_id);
+out:
+	i915_gem_context_put(ctx);
 	return 0;
 }
 
@@ -1044,17 +1043,11 @@ int i915_gem_context_getparam_ioctl(struct drm_device *dev, void *data,
 	struct drm_i915_file_private *file_priv = file->driver_priv;
 	struct drm_i915_gem_context_param *args = data;
 	struct i915_gem_context *ctx;
-	int ret;
-
-	ret = i915_mutex_lock_interruptible(dev);
-	if (ret)
-		return ret;
+	int ret = 0;
 
 	ctx = i915_gem_context_lookup(file_priv, args->ctx_id);
-	if (IS_ERR(ctx)) {
-		mutex_unlock(&dev->struct_mutex);
-		return PTR_ERR(ctx);
-	}
+	if (!ctx)
+		return -ENOENT;
 
 	args->size = 0;
 	switch (args->param) {
@@ -1082,8 +1075,8 @@ int i915_gem_context_getparam_ioctl(struct drm_device *dev, void *data,
 		ret = -EINVAL;
 		break;
 	}
-	mutex_unlock(&dev->struct_mutex);
 
+	i915_gem_context_put(ctx);
 	return ret;
 }
 
@@ -1095,15 +1088,13 @@ int i915_gem_context_setparam_ioctl(struct drm_device *dev, void *data,
 	struct i915_gem_context *ctx;
 	int ret;
 
+	ctx = i915_gem_context_lookup(file_priv, args->ctx_id);
+	if (!ctx)
+		return -ENOENT;
+
 	ret = i915_mutex_lock_interruptible(dev);
 	if (ret)
-		return ret;
-
-	ctx = i915_gem_context_lookup(file_priv, args->ctx_id);
-	if (IS_ERR(ctx)) {
-		mutex_unlock(&dev->struct_mutex);
-		return PTR_ERR(ctx);
-	}
+		goto out;
 
 	switch (args->param) {
 	case I915_CONTEXT_PARAM_BAN_PERIOD:
@@ -1141,6 +1132,8 @@ int i915_gem_context_setparam_ioctl(struct drm_device *dev, void *data,
 	}
 	mutex_unlock(&dev->struct_mutex);
 
+out:
+	i915_gem_context_put(ctx);
 	return ret;
 }
 
@@ -1155,27 +1148,31 @@ int i915_gem_context_reset_stats_ioctl(struct drm_device *dev,
 	if (args->flags || args->pad)
 		return -EINVAL;
 
-	ret = i915_mutex_lock_interruptible(dev);
-	if (ret)
-		return ret;
+	ret = -ENOENT;
+	rcu_read_lock();
+	ctx = __i915_gem_context_lookup_rcu(file->driver_priv, args->ctx_id);
+	if (!ctx)
+		goto out;
 
-	ctx = i915_gem_context_lookup(file->driver_priv, args->ctx_id);
-	if (IS_ERR(ctx)) {
-		mutex_unlock(&dev->struct_mutex);
-		return PTR_ERR(ctx);
-	}
+	/*
+	 * We opt for unserialised reads here. This may result in tearing
+	 * in the extremely unlikely event of a GPU hang on this context
+	 * as we are querying them. If we need that extra layer of protection,
+	 * we should wrap the hangstats with a seqlock.
+	 */
 
 	if (capable(CAP_SYS_ADMIN))
 		args->reset_count = i915_reset_count(&dev_priv->gpu_error);
 	else
 		args->reset_count = 0;
 
-	args->batch_active = ctx->guilty_count;
-	args->batch_pending = ctx->active_count;
-
-	mutex_unlock(&dev->struct_mutex);
+	args->batch_active = READ_ONCE(ctx->guilty_count);
+	args->batch_pending = READ_ONCE(ctx->active_count);
 
-	return 0;
+	ret = 0;
+out:
+	rcu_read_unlock();
+	return ret;
 }
 
 #if IS_ENABLED(CONFIG_DRM_I915_SELFTEST)

drivers/gpu/drm/i915/i915_gem_context.h

@@ -99,6 +99,11 @@ struct i915_gem_context {
 	 */
 	struct kref ref;
 
+	/**
+	 * @rcu: rcu_head for deferred freeing.
+	 */
+	struct rcu_head rcu;
+
 	/**
 	 * @flags: small set of booleans
 	 */

drivers/gpu/drm/i915/i915_gem_execbuffer.c

@@ -669,16 +669,17 @@ static int eb_select_context(struct i915_execbuffer *eb)
 	struct i915_gem_context *ctx;
 
 	ctx = i915_gem_context_lookup(eb->file->driver_priv, eb->args->rsvd1);
-	if (unlikely(IS_ERR(ctx)))
-		return PTR_ERR(ctx);
+	if (unlikely(!ctx))
+		return -ENOENT;
 
 	if (unlikely(i915_gem_context_is_banned(ctx))) {
 		DRM_DEBUG("Context %u tried to submit while banned\n",
 			  ctx->user_handle);
+		i915_gem_context_put(ctx);
 		return -EIO;
 	}
 
-	eb->ctx = i915_gem_context_get(ctx);
+	eb->ctx = ctx;
 	eb->vm = ctx->ppgtt ? &ctx->ppgtt->base : &eb->i915->ggtt.base;
 
 	eb->context_flags = 0;
@@ -2127,7 +2128,6 @@ i915_gem_do_execbuffer(struct drm_device *dev,
 	if (DBG_FORCE_RELOC || !(args->flags & I915_EXEC_NO_RELOC))
 		args->flags |= __EXEC_HAS_RELOC;
 	eb.exec = exec;
-	eb.ctx = NULL;
 	eb.invalid_flags = __EXEC_OBJECT_UNKNOWN_FLAGS;
 	if (USES_FULL_PPGTT(eb.i915))
 		eb.invalid_flags |= EXEC_OBJECT_NEEDS_GTT;
@@ -2182,6 +2182,10 @@ i915_gem_do_execbuffer(struct drm_device *dev,
 	if (eb_create(&eb))
 		return -ENOMEM;
 
+	err = eb_select_context(&eb);
+	if (unlikely(err))
+		goto err_destroy;
+
 	/*
 	 * Take a local wakeref for preparing to dispatch the execbuf as
 	 * we expect to access the hardware fairly frequently in the
@@ -2190,14 +2194,11 @@ i915_gem_do_execbuffer(struct drm_device *dev,
 	 * 100ms.
 	 */
 	intel_runtime_pm_get(eb.i915);
+
 	err = i915_mutex_lock_interruptible(dev);
 	if (err)
 		goto err_rpm;
 
-	err = eb_select_context(&eb);
-	if (unlikely(err))
-		goto err_unlock;
-
 	err = eb_relocate(&eb);
 	if (err)
 		/*
@@ -2333,11 +2334,11 @@ i915_gem_do_execbuffer(struct drm_device *dev,
 err_vma:
 	if (eb.exec)
 		eb_release_vmas(&eb);
-	i915_gem_context_put(eb.ctx);
-err_unlock:
 	mutex_unlock(&dev->struct_mutex);
 err_rpm:
 	intel_runtime_pm_put(eb.i915);
+	i915_gem_context_put(eb.ctx);
+err_destroy:
 	eb_destroy(&eb);
 	if (out_fence_fd != -1)
 		put_unused_fd(out_fence_fd);