Re: [Intel-gfx] [PATCH v2] drm/i915: Fix race in __i915_vma_remove_closed

[Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxxxxxxxx>]

On 20/04/2022 10:57, Karol Herbst wrote:
> i915_vma_reopen checked if the vma is closed before without taking the
> lock. So multiple threads could attempt removing the vma.
>
> Instead the lock needs to be taken before actually checking.
>
> v2: move struct declaration

Fix looks correct to me. In which case it seems breakage was introduced 
with:

Fixes: 155ab8836caa ("drm/i915: Move object close under its own lock")
Cc: Matthew Auld <matthew.auld@xxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx> # v5.3+

AFAICT at least. I will add these tags and pull it in unless someone 
shouts a correction.

Regards,

Tvrtko

> Cc: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
> Cc: intel-gfx@xxxxxxxxxxxxxxxxxxxxx
> Cc: dri-devel@xxxxxxxxxxxxxxxxxxxxx
> Closes: https://gitlab.freedesktop.org/drm/intel/-/issues/5732
> Signed-off-by: Karol Herbst <kherbst@xxxxxxxxxx>
> ---
>   drivers/gpu/drm/i915/i915_vma.c | 11 +++++++----
>   1 file changed, 7 insertions(+), 4 deletions(-)
>
> diff --git a/drivers/gpu/drm/i915/i915_vma.c b/drivers/gpu/drm/i915/i915_vma.c
> index 162e8d83691b..2efdad2b43fa 100644
> --- a/drivers/gpu/drm/i915/i915_vma.c
> +++ b/drivers/gpu/drm/i915/i915_vma.c
> @@ -1615,17 +1615,17 @@ void i915_vma_close(struct i915_vma *vma)
>   
>   static void __i915_vma_remove_closed(struct i915_vma *vma)
>   {
> -	struct intel_gt *gt = vma->vm->gt;
> -
> -	spin_lock_irq(&gt->closed_lock);
>   	list_del_init(&vma->closed_link);
> -	spin_unlock_irq(&gt->closed_lock);
>   }
>   
>   void i915_vma_reopen(struct i915_vma *vma)
>   {
> +	struct intel_gt *gt = vma->vm->gt;
> +
> +	spin_lock_irq(&gt->closed_lock);
>   	if (i915_vma_is_closed(vma))
>   		__i915_vma_remove_closed(vma);
> +	spin_unlock_irq(&gt->closed_lock);
>   }
>   
>   static void force_unbind(struct i915_vma *vma)
> @@ -1641,6 +1641,7 @@ static void force_unbind(struct i915_vma *vma)
>   static void release_references(struct i915_vma *vma, bool vm_ddestroy)
>   {
>   	struct drm_i915_gem_object *obj = vma->obj;
> +	struct intel_gt *gt = vma->vm->gt;
>   
>   	GEM_BUG_ON(i915_vma_is_active(vma));
>   
> @@ -1651,7 +1652,9 @@ static void release_references(struct i915_vma *vma, bool vm_ddestroy)
>   
>   	spin_unlock(&obj->vma.lock);
>   
> +	spin_lock_irq(&gt->closed_lock);
>   	__i915_vma_remove_closed(vma);
> +	spin_unlock_irq(&gt->closed_lock);
>   
>   	if (vm_ddestroy)
>   		i915_vm_resv_put(vma->vm);