Re: [PATCH 1/6] drm/ttm: change fence_lock to inner lock

[Thomas Hellstrom <thellstrom@vmware.com>]

On 11/28/2012 12:25 PM, Maarten Lankhorst wrote:
> This requires changing the order in ttm_bo_cleanup_refs_or_queue to
> take the reservation first, as there is otherwise no race free way to
> take lru lock before fence_lock.
>
> Signed-off-by: Maarten Lankhorst <maarten.lankhorst@canonical.com>

Technically I think it would be possible to avoid reserving in the case 
the bo is still busy, and re-checking
for sync_obj presence once we have reserved (since we release the fence 
lock).

But that's an optimization. The important thing at this point is to get 
this right.

Reviewed-by: Thomas Hellstrom <thellstrom@vmware.com>



> ---
>   drivers/gpu/drm/ttm/ttm_bo.c           | 31 +++++++++++--------------------
>   drivers/gpu/drm/ttm/ttm_execbuf_util.c |  4 ++--
>   2 files changed, 13 insertions(+), 22 deletions(-)
>
> diff --git a/drivers/gpu/drm/ttm/ttm_bo.c b/drivers/gpu/drm/ttm/ttm_bo.c
> index 7426fe5..202fc20 100644
> --- a/drivers/gpu/drm/ttm/ttm_bo.c
> +++ b/drivers/gpu/drm/ttm/ttm_bo.c
> @@ -500,27 +500,17 @@ static void ttm_bo_cleanup_refs_or_queue(struct ttm_buffer_object *bo)
>   {
>   	struct ttm_bo_device *bdev = bo->bdev;
>   	struct ttm_bo_global *glob = bo->glob;
> -	struct ttm_bo_driver *driver;
> +	struct ttm_bo_driver *driver = bdev->driver;
>   	void *sync_obj = NULL;
>   	int put_count;
>   	int ret;
>   
> +	spin_lock(&glob->lru_lock);
> +	ret = ttm_bo_reserve_locked(bo, false, true, false, 0);
> +
>   	spin_lock(&bdev->fence_lock);
>   	(void) ttm_bo_wait(bo, false, false, true);
> -	if (!bo->sync_obj) {
> -
> -		spin_lock(&glob->lru_lock);
> -
> -		/**
> -		 * Lock inversion between bo:reserve and bdev::fence_lock here,
> -		 * but that's OK, since we're only trylocking.
> -		 */
> -
> -		ret = ttm_bo_reserve_locked(bo, false, true, false, 0);
> -
> -		if (unlikely(ret == -EBUSY))
> -			goto queue;
> -
> +	if (!ret && !bo->sync_obj) {
>   		spin_unlock(&bdev->fence_lock);
>   		put_count = ttm_bo_del_from_lru(bo);
>   
> @@ -530,18 +520,19 @@ static void ttm_bo_cleanup_refs_or_queue(struct ttm_buffer_object *bo)
>   		ttm_bo_list_ref_sub(bo, put_count, true);
>   
>   		return;
> -	} else {
> -		spin_lock(&glob->lru_lock);
>   	}
> -queue:
> -	driver = bdev->driver;
>   	if (bo->sync_obj)
>   		sync_obj = driver->sync_obj_ref(bo->sync_obj);
> +	spin_unlock(&bdev->fence_lock);
> +
> +	if (!ret) {
> +		atomic_set(&bo->reserved, 0);
> +		wake_up_all(&bo->event_queue);
> +	}
>   
>   	kref_get(&bo->list_kref);
>   	list_add_tail(&bo->ddestroy, &bdev->ddestroy);
>   	spin_unlock(&glob->lru_lock);
> -	spin_unlock(&bdev->fence_lock);
>   
>   	if (sync_obj) {
>   		driver->sync_obj_flush(sync_obj);
> diff --git a/drivers/gpu/drm/ttm/ttm_execbuf_util.c b/drivers/gpu/drm/ttm/ttm_execbuf_util.c
> index 1986d00..cd9e452 100644
> --- a/drivers/gpu/drm/ttm/ttm_execbuf_util.c
> +++ b/drivers/gpu/drm/ttm/ttm_execbuf_util.c
> @@ -213,8 +213,8 @@ void ttm_eu_fence_buffer_objects(struct list_head *list, void *sync_obj)
>   	driver = bdev->driver;
>   	glob = bo->glob;
>   
> -	spin_lock(&bdev->fence_lock);
>   	spin_lock(&glob->lru_lock);
> +	spin_lock(&bdev->fence_lock);
>   
>   	list_for_each_entry(entry, list, head) {
>   		bo = entry->bo;
> @@ -223,8 +223,8 @@ void ttm_eu_fence_buffer_objects(struct list_head *list, void *sync_obj)
>   		ttm_bo_unreserve_locked(bo);
>   		entry->reserved = false;
>   	}
> -	spin_unlock(&glob->lru_lock);
>   	spin_unlock(&bdev->fence_lock);
> +	spin_unlock(&glob->lru_lock);
>   
>   	list_for_each_entry(entry, list, head) {
>   		if (entry->old_sync_obj)