drm/vmwgfx: Remove rcu locks from user resources

User resource lookups used rcu to avoid two extra atomics. Unfortunately
the rcu paths were buggy and it was easy to make the driver crash by
submitting command buffers from two different threads. Because the
lookups never show up in performance profiles replace them with a
regular spin lock which fixes the races in accesses to those shared
resources.

Fixes kernel oops'es in IGT's vmwgfx execution_buffer stress test and
seen crashes with apps using shared resources.

Fixes: e14c02e6 ("drm/vmwgfx: Look up objects without taking a reference")
Signed-off-by: Zack Rusin <zackr@vmware.com>
Reviewed-by: Martin Krastev <krastevm@vmware.com>
Reviewed-by: Maaz Mombasawala <mombasawalam@vmware.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221207172907.959037-1-zack@kde.org

drivers/gpu/drm/vmwgfx/ttm_object.c

@@ -254,40 +254,6 @@ void ttm_base_object_unref(struct ttm_base_object **p_base)
 	kref_put(&base->refcount, ttm_release_base);
 }
 
-/**
- * ttm_base_object_noref_lookup - look up a base object without reference
- * @tfile: The struct ttm_object_file the object is registered with.
- * @key: The object handle.
- *
- * This function looks up a ttm base object and returns a pointer to it
- * without refcounting the pointer. The returned pointer is only valid
- * until ttm_base_object_noref_release() is called, and the object
- * pointed to by the returned pointer may be doomed. Any persistent usage
- * of the object requires a refcount to be taken using kref_get_unless_zero().
- * Iff this function returns successfully it needs to be paired with
- * ttm_base_object_noref_release() and no sleeping- or scheduling functions
- * may be called inbetween these function callse.
- *
- * Return: A pointer to the object if successful or NULL otherwise.
- */
-struct ttm_base_object *
-ttm_base_object_noref_lookup(struct ttm_object_file *tfile, uint64_t key)
-{
-	struct vmwgfx_hash_item *hash;
-	int ret;
-
-	rcu_read_lock();
-	ret = ttm_tfile_find_ref_rcu(tfile, key, &hash);
-	if (ret) {
-		rcu_read_unlock();
-		return NULL;
-	}
-
-	__release(RCU);
-	return hlist_entry(hash, struct ttm_ref_object, hash)->obj;
-}
-EXPORT_SYMBOL(ttm_base_object_noref_lookup);
-
 struct ttm_base_object *ttm_base_object_lookup(struct ttm_object_file *tfile,
 					       uint64_t key)
 {
@@ -295,15 +261,16 @@ struct ttm_base_object *ttm_base_object_lookup(struct ttm_object_file *tfile,
 	struct vmwgfx_hash_item *hash;
 	int ret;
 
-	rcu_read_lock();
-	ret = ttm_tfile_find_ref_rcu(tfile, key, &hash);
+	spin_lock(&tfile->lock);
+	ret = ttm_tfile_find_ref(tfile, key, &hash);
 
 	if (likely(ret == 0)) {
 		base = hlist_entry(hash, struct ttm_ref_object, hash)->obj;
 		if (!kref_get_unless_zero(&base->refcount))
 			base = NULL;
 	}
-	rcu_read_unlock();
+	spin_unlock(&tfile->lock);
+
 
 	return base;
 }

drivers/gpu/drm/vmwgfx/ttm_object.h

@@ -307,18 +307,4 @@ extern int ttm_prime_handle_to_fd(struct ttm_object_file *tfile,
 #define ttm_prime_object_kfree(__obj, __prime)		\
 	kfree_rcu(__obj, __prime.base.rhead)
 
-struct ttm_base_object *
-ttm_base_object_noref_lookup(struct ttm_object_file *tfile, uint64_t key);
-
-/**
- * ttm_base_object_noref_release - release a base object pointer looked up
- * without reference
- *
- * Releases a base object pointer looked up with ttm_base_object_noref_lookup().
- */
-static inline void ttm_base_object_noref_release(void)
-{
-	__acquire(RCU);
-	rcu_read_unlock();
-}
 #endif

drivers/gpu/drm/vmwgfx/vmwgfx_bo.c

@@ -715,44 +715,6 @@ int vmw_user_bo_lookup(struct drm_file *filp,
 	return 0;
 }
 
-/**
- * vmw_user_bo_noref_lookup - Look up a vmw user buffer object without reference
- * @filp: The TTM object file the handle is registered with.
- * @handle: The user buffer object handle.
- *
- * This function looks up a struct vmw_bo and returns a pointer to the
- * struct vmw_buffer_object it derives from without refcounting the pointer.
- * The returned pointer is only valid until vmw_user_bo_noref_release() is
- * called, and the object pointed to by the returned pointer may be doomed.
- * Any persistent usage of the object requires a refcount to be taken using
- * ttm_bo_reference_unless_doomed(). Iff this function returns successfully it
- * needs to be paired with vmw_user_bo_noref_release() and no sleeping-
- * or scheduling functions may be called in between these function calls.
- *
- * Return: A struct vmw_buffer_object pointer if successful or negative
- * error pointer on failure.
- */
-struct vmw_buffer_object *
-vmw_user_bo_noref_lookup(struct drm_file *filp, u32 handle)
-{
-	struct vmw_buffer_object *vmw_bo;
-	struct ttm_buffer_object *bo;
-	struct drm_gem_object *gobj = drm_gem_object_lookup(filp, handle);
-
-	if (!gobj) {
-		DRM_ERROR("Invalid buffer object handle 0x%08lx.\n",
-			  (unsigned long)handle);
-		return ERR_PTR(-ESRCH);
-	}
-	vmw_bo = gem_to_vmw_bo(gobj);
-	bo = ttm_bo_get_unless_zero(&vmw_bo->base);
-	vmw_bo = vmw_buffer_object(bo);
-	drm_gem_object_put(gobj);
-
-	return vmw_bo;
-}
-
-
 /**
  * vmw_bo_fence_single - Utility function to fence a single TTM buffer
  *                       object without unreserving it.

drivers/gpu/drm/vmwgfx/vmwgfx_drv.h

@@ -830,12 +830,7 @@ extern int vmw_user_resource_lookup_handle(
 	uint32_t handle,
 	const struct vmw_user_resource_conv *converter,
 	struct vmw_resource **p_res);
-extern struct vmw_resource *
-vmw_user_resource_noref_lookup_handle(struct vmw_private *dev_priv,
-				      struct ttm_object_file *tfile,
-				      uint32_t handle,
-				      const struct vmw_user_resource_conv *
-				      converter);
+
 extern int vmw_stream_claim_ioctl(struct drm_device *dev, void *data,
 				  struct drm_file *file_priv);
 extern int vmw_stream_unref_ioctl(struct drm_device *dev, void *data,
@@ -874,15 +869,6 @@ static inline bool vmw_resource_mob_attached(const struct vmw_resource *res)
 	return !RB_EMPTY_NODE(&res->mob_node);
 }
 
-/**
- * vmw_user_resource_noref_release - release a user resource pointer looked up
- * without reference
- */
-static inline void vmw_user_resource_noref_release(void)
-{
-	ttm_base_object_noref_release();
-}
-
 /**
  * Buffer object helper functions - vmwgfx_bo.c
  */
@@ -934,8 +920,6 @@ extern void vmw_bo_unmap(struct vmw_buffer_object *vbo);
 extern void vmw_bo_move_notify(struct ttm_buffer_object *bo,
 			       struct ttm_resource *mem);
 extern void vmw_bo_swap_notify(struct ttm_buffer_object *bo);
-extern struct vmw_buffer_object *
-vmw_user_bo_noref_lookup(struct drm_file *filp, u32 handle);
 
 /**
  * vmw_bo_adjust_prio - Adjust the buffer object eviction priority

drivers/gpu/drm/vmwgfx/vmwgfx_resource.c

@@ -281,39 +281,6 @@ int vmw_user_resource_lookup_handle(struct vmw_private *dev_priv,
 	return ret;
 }
 
-/**
- * vmw_user_resource_noref_lookup_handle - lookup a struct resource from a
- * TTM user-space handle and perform basic type checks
- *
- * @dev_priv:     Pointer to a device private struct
- * @tfile:        Pointer to a struct ttm_object_file identifying the caller
- * @handle:       The TTM user-space handle
- * @converter:    Pointer to an object describing the resource type
- *
- * If the handle can't be found or is associated with an incorrect resource
- * type, -EINVAL will be returned.
- */
-struct vmw_resource *
-vmw_user_resource_noref_lookup_handle(struct vmw_private *dev_priv,
-				      struct ttm_object_file *tfile,
-				      uint32_t handle,
-				      const struct vmw_user_resource_conv
-				      *converter)
-{
-	struct ttm_base_object *base;
-
-	base = ttm_base_object_noref_lookup(tfile, handle);
-	if (!base)
-		return ERR_PTR(-ESRCH);
-
-	if (unlikely(ttm_base_object_type(base) != converter->object_type)) {
-		ttm_base_object_noref_release();
-		return ERR_PTR(-EINVAL);
-	}
-
-	return converter->base_obj_to_res(base);
-}
-
 /*
  * Helper function that looks either a surface or bo.
  *