Re: [PATCH] drm/amdgpu: add rcu_barrier after entity fini

["Christian König" <ckoenig.leichtzumerken-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>]

Am 17.05.2018 um 12:03 schrieb Emily Deng:
> To free the fence from the amdgpu_fence_slab, need twice call_rcu, to avoid
> the amdgpu_fence_slab_fini call kmem_cache_destroy(amdgpu_fence_slab) before
> kmem_cache_free(amdgpu_fence_slab, fence), add rcu_barrier after drm_sched_entity_fini.
>
> The kmem_cache_free(amdgpu_fence_slab, fence)'s call trace as below:
> 1.drm_sched_entity_fini ->
> drm_sched_entity_cleanup ->
> dma_fence_put(entity->last_scheduled) ->
> drm_sched_fence_release_finished ->
> drm_sched_fence_release_scheduled ->
> call_rcu(&fence->finished.rcu, drm_sched_fence_free)
>
> 2.drm_sched_fence_free ->
> dma_fence_put(fence->parent) ->
> amdgpu_fence_release ->
> call_rcu(&f->rcu, amdgpu_fence_free) ->
> kmem_cache_free(amdgpu_fence_slab, fence);
>
> v2:put the barrier before the kmem_cache_destroy
>
> Change-Id: I8dcadd3372f97e72461bf46b41cc26d90f09b8df
> Signed-off-by: Emily Deng <Emily.Deng@amd.com>
> ---
>   drivers/gpu/drm/amd/amdgpu/amdgpu_fence.c | 1 +
>   1 file changed, 1 insertion(+)
>
> diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_fence.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_fence.c
> index 39ec6b8..42be65b 100644
> --- a/drivers/gpu/drm/amd/amdgpu/amdgpu_fence.c
> +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_fence.c
> @@ -69,6 +69,7 @@ int amdgpu_fence_slab_init(void)
>   void amdgpu_fence_slab_fini(void)
>   {
>   	rcu_barrier();
> +	rcu_barrier();

Well, you should have noted that there is already an rcu_barrier here 
and adding another one shouldn't have any additional effect. So your 
explanation and the proposed solution doesn't make to much sense.

I think the problem you run into is rather that the fence is reference 
counted and might live longer than the module who created it.

Complicated issue, one possible solution would be to release 
fence->parent earlier in the scheduler fence but that doesn't sound like 
a general purpose solution.

Christian.

>   	kmem_cache_destroy(amdgpu_fence_slab);
>   }
>   /*

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