Re: [PATCH 5/5] drm/mm: Use clear_bit_unlock() for releasing the drm_mm_node()

[Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>]

On 03/10/2019 22:01, Chris Wilson wrote:
> A few callers need to serialise the destruction of their drm_mm_node and
> ensure it is removed from the drm_mm before freeing. However, to be
> completely sure that any access from another thread is complete before
> we free the struct, we require the RELEASE semantics of
> clear_bit_unlock().
> 
> This allows the conditional locking such as
> 
> Thread A				Thread B
>      mutex_lock(mm_lock);                if (drm_mm_node_allocated(node)) {
>      drm_mm_node_remove(node);               mutex_lock(mm_lock);
>      mutex_unlock(mm_lock);                  drm_mm_node_remove(node);
>                                              mutex_unlock(mm_lock);
>                                           }
>                                           kfree(node);

My understanding is that release semantics on node allocated mean 1 -> 0 
transition is guaranteed to be seen only when thread A 
drm_mm_node_remove is fully done with the removal. But if it is in the 
middle of removal, node is still seen as allocated outside and thread B 
can enter the if-body, wait for the lock, and then drm_mm_node_remove 
will attempt a double removal. So I think another drm_mm_node_allocated 
under the lock is needed.

But the fkree part looks correct. There can be no false negatives with 
the release semantics on the allocated bit.

Regards,

Tvrtko

> to serialise correctly without any lingering accesses from A to the
> freed node. Allocation / insertion of the node is assumed never to race
> with removal or eviction scanning.
> 
> Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
> ---
>   drivers/gpu/drm/drm_mm.c | 11 ++++++-----
>   1 file changed, 6 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/gpu/drm/drm_mm.c b/drivers/gpu/drm/drm_mm.c
> index a9cab5e53731..2a6e34663146 100644
> --- a/drivers/gpu/drm/drm_mm.c
> +++ b/drivers/gpu/drm/drm_mm.c
> @@ -424,9 +424,9 @@ int drm_mm_reserve_node(struct drm_mm *mm, struct drm_mm_node *node)
>   
>   	node->mm = mm;
>   
> +	__set_bit(DRM_MM_NODE_ALLOCATED_BIT, &node->flags);
>   	list_add(&node->node_list, &hole->node_list);
>   	drm_mm_interval_tree_add_node(hole, node);
> -	__set_bit(DRM_MM_NODE_ALLOCATED_BIT, &node->flags);
>   	node->hole_size = 0;
>   
>   	rm_hole(hole);
> @@ -543,9 +543,9 @@ int drm_mm_insert_node_in_range(struct drm_mm * const mm,
>   		node->color = color;
>   		node->hole_size = 0;
>   
> +		__set_bit(DRM_MM_NODE_ALLOCATED_BIT, &node->flags);
>   		list_add(&node->node_list, &hole->node_list);
>   		drm_mm_interval_tree_add_node(hole, node);
> -		__set_bit(DRM_MM_NODE_ALLOCATED_BIT, &node->flags);
>   
>   		rm_hole(hole);
>   		if (adj_start > hole_start)
> @@ -589,11 +589,12 @@ void drm_mm_remove_node(struct drm_mm_node *node)
>   
>   	drm_mm_interval_tree_remove(node, &mm->interval_tree);
>   	list_del(&node->node_list);
> -	__clear_bit(DRM_MM_NODE_ALLOCATED_BIT, &node->flags);
>   
>   	if (drm_mm_hole_follows(prev_node))
>   		rm_hole(prev_node);
>   	add_hole(prev_node);
> +
> +	clear_bit_unlock(DRM_MM_NODE_ALLOCATED_BIT, &node->flags);
>   }
>   EXPORT_SYMBOL(drm_mm_remove_node);
>   
> @@ -614,6 +615,7 @@ void drm_mm_replace_node(struct drm_mm_node *old, struct drm_mm_node *new)
>   
>   	*new = *old;
>   
> +	__set_bit(DRM_MM_NODE_ALLOCATED_BIT, &new->flags);
>   	list_replace(&old->node_list, &new->node_list);
>   	rb_replace_node_cached(&old->rb, &new->rb, &mm->interval_tree);
>   
> @@ -627,8 +629,7 @@ void drm_mm_replace_node(struct drm_mm_node *old, struct drm_mm_node *new)
>   				&mm->holes_addr);
>   	}
>   
> -	__clear_bit(DRM_MM_NODE_ALLOCATED_BIT, &old->flags);
> -	__set_bit(DRM_MM_NODE_ALLOCATED_BIT, &new->flags);
> +	clear_bit_unlock(DRM_MM_NODE_ALLOCATED_BIT, &old->flags);
>   }
>   EXPORT_SYMBOL(drm_mm_replace_node);
>   
> 
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