Re: [PATCH] drm/xe: Fix missing runtime outer protection for ggtt_remove_node

["Thomas Hellström" <thomas.hellstrom@linux.intel.com>]

On Fri, 2024-05-31 at 16:08 -0400, Rodrigo Vivi wrote:
> Defer the ggtt node removal to a thread if runtime_pm is not active.
> 
> The ggtt node removal can be called from multiple places, including
> places where we cannot protect with outer callers and places we are
> within other locks. So, try to grab the runtime reference if the
> device is already active, otherwise defer the removal to a separate
> thread from where we are sure we can wake the device up.
> 
> v2: - use xe wq instead of system wq (Matt and CI)
>     - Avoid GFP_KERNEL to be future proof since this removal can
>     be called from outside our drivers and we don't want to block
>     if atomic is needed. (Matt)
> v3: amend forgot chunk declaring xe_device.
> 
> Cc: Paulo Zanoni <paulo.r.zanoni@intel.com>
> Cc: Francois Dugast <francois.dugast@intel.com>
> Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
> Cc: Matthew Brost <matthew.brost@intel.com>
> Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
> ---
>  drivers/gpu/drm/xe/xe_ggtt.c | 57 ++++++++++++++++++++++++++++++++--
> --
>  1 file changed, 52 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/gpu/drm/xe/xe_ggtt.c
> b/drivers/gpu/drm/xe/xe_ggtt.c
> index b01a670fecb8..ab086737f910 100644
> --- a/drivers/gpu/drm/xe/xe_ggtt.c
> +++ b/drivers/gpu/drm/xe/xe_ggtt.c
> @@ -443,16 +443,14 @@ int xe_ggtt_insert_bo(struct xe_ggtt *ggtt,
> struct xe_bo *bo)
>  	return __xe_ggtt_insert_bo_at(ggtt, bo, 0, U64_MAX);
>  }
>  
> -void xe_ggtt_remove_node(struct xe_ggtt *ggtt, struct drm_mm_node
> *node,
> -			 bool invalidate)
> +static void ggtt_remove_node(struct xe_ggtt *ggtt, struct
> drm_mm_node *node,
> +			     bool invalidate)
>  {
>  	struct xe_device *xe = tile_to_xe(ggtt->tile);
>  	bool bound;
>  	int idx;
>  
>  	bound = drm_dev_enter(&xe->drm, &idx);
> -	if (bound)
> -		xe_pm_runtime_get_noresume(xe);
>  
>  	mutex_lock(&ggtt->lock);
>  	if (bound)
> @@ -467,10 +465,59 @@ void xe_ggtt_remove_node(struct xe_ggtt *ggtt,
> struct drm_mm_node *node,
>  	if (invalidate)
>  		xe_ggtt_invalidate(ggtt);
>  
> -	xe_pm_runtime_put(xe);
>  	drm_dev_exit(idx);
>  }
>  
> +struct remove_node_work {
> +	struct work_struct work;
> +	struct xe_ggtt *ggtt;
> +	struct drm_mm_node *node;
> +	bool invalidate;
> +};
> +
> +static void ggtt_remove_node_work_func(struct work_struct *work)
> +{
> +	struct remove_node_work *remove_node = container_of(work,
> struct remove_node_work, work);
> +	struct xe_device *xe = tile_to_xe(remove_node->ggtt->tile);
> +
> +	xe_pm_runtime_get(xe);
> +	ggtt_remove_node(remove_node->ggtt, remove_node->node,
> remove_node->invalidate);
> +	xe_pm_runtime_put(xe);
> +
> +	kfree(remove_node);
> +}
> +
> +static void ggtt_queue_remove_node(struct xe_ggtt *ggtt, struct
> drm_mm_node *node,
> +				   bool invalidate)
> +{
> +	struct xe_device *xe = tile_to_xe(ggtt->tile);
> +	struct remove_node_work *remove_node;
> +
> +	remove_node = kmalloc(sizeof(*remove_node), GFP_ATOMIC);
> +	if (!remove_node)
> +		return;
> +
> +	INIT_WORK(&remove_node->work, ggtt_remove_node_work_func);
> +	remove_node->ggtt = ggtt;
> +	remove_node->node = node;
> +	remove_node->invalidate = invalidate;
> +
> +	queue_work(xe->unordered_wq, &remove_node->work);

I don't see anything guaranteeing that there are no work items left at
ggtt fini. The workqueue isn't flushed until xe_device_destroy(),
which, from what I can tell, happens *after* ggtt_fini_early().

/Thomas




> +}
> +
> +void xe_ggtt_remove_node(struct xe_ggtt *ggtt, struct drm_mm_node
> *node,
> +			 bool invalidate)
> +{
> +	struct xe_device *xe = tile_to_xe(ggtt->tile);
> +
> +	if (xe_pm_runtime_get_if_active(xe)) {
> +		ggtt_remove_node(ggtt, node, invalidate);
> +		xe_pm_runtime_put(xe);
> +	} else {
> +		ggtt_queue_remove_node(ggtt, node, invalidate);
> +	}
> +}
> +
>  void xe_ggtt_remove_bo(struct xe_ggtt *ggtt, struct xe_bo *bo)
>  {
>  	if (XE_WARN_ON(!bo->ggtt_node.size))