Re: [PATCH] drm/amdgpu: Fix potential race processing vm->freed

["Christian König" <christian.koenig@amd.com>]

Am 03.02.23 um 19:10 schrieb Rob Clark:
> From: Rob Clark <robdclark@chromium.org>
>
> If userspace calls the AMDGPU_CS ioctl from multiple threads, because
> the vm is global to the drm_file, you can end up with multiple threads
> racing in amdgpu_vm_clear_freed().  So the freed list should be
> protected with the status_lock, similar to other vm lists.

Well this is nonsense. To process the freed list the VM root PD lock 
must be held anyway.

If we have a call path where this isn't true then we have a major bug at 
a different place here.

Regards,
Christian.

>
> Fixes: d38ceaf99ed0 ("drm/amdgpu: add core driver (v4)")
> Signed-off-by: Rob Clark <robdclark@chromium.org>
> ---
>   drivers/gpu/drm/amd/amdgpu/amdgpu_vm.c | 33 ++++++++++++++++++++++----
>   1 file changed, 29 insertions(+), 4 deletions(-)
>
> diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_vm.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_vm.c
> index b9441ab457ea..aeed7bc1512f 100644
> --- a/drivers/gpu/drm/amd/amdgpu/amdgpu_vm.c
> +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_vm.c
> @@ -1240,10 +1240,19 @@ int amdgpu_vm_clear_freed(struct amdgpu_device *adev,
>   	struct amdgpu_bo_va_mapping *mapping;
>   	uint64_t init_pte_value = 0;
>   	struct dma_fence *f = NULL;
> +	struct list_head freed;
>   	int r;
>   
> -	while (!list_empty(&vm->freed)) {
> -		mapping = list_first_entry(&vm->freed,
> +	/*
> +	 * Move the contents of the VM's freed list to a local list
> +	 * that we can iterate without racing against other threads:
> +	 */
> +	spin_lock(&vm->status_lock);
> +	list_replace_init(&vm->freed, &freed);
> +	spin_unlock(&vm->status_lock);
> +
> +	while (!list_empty(&freed)) {
> +		mapping = list_first_entry(&freed,
>   			struct amdgpu_bo_va_mapping, list);
>   		list_del(&mapping->list);
>   
> @@ -1258,6 +1267,15 @@ int amdgpu_vm_clear_freed(struct amdgpu_device *adev,
>   		amdgpu_vm_free_mapping(adev, vm, mapping, f);
>   		if (r) {
>   			dma_fence_put(f);
> +
> +			/*
> +			 * Move any unprocessed mappings back to the freed
> +			 * list:
> +			 */
> +			spin_lock(&vm->status_lock);
> +			list_splice_tail(&freed, &vm->freed);
> +			spin_unlock(&vm->status_lock);
> +
>   			return r;
>   		}
>   	}
> @@ -1583,11 +1601,14 @@ int amdgpu_vm_bo_unmap(struct amdgpu_device *adev,
>   	mapping->bo_va = NULL;
>   	trace_amdgpu_vm_bo_unmap(bo_va, mapping);
>   
> -	if (valid)
> +	if (valid) {
> +		spin_lock(&vm->status_lock);
>   		list_add(&mapping->list, &vm->freed);
> -	else
> +		spin_unlock(&vm->status_lock);
> +	} else {
>   		amdgpu_vm_free_mapping(adev, vm, mapping,
>   				       bo_va->last_pt_update);
> +	}
>   
>   	return 0;
>   }
> @@ -1671,7 +1692,9 @@ int amdgpu_vm_bo_clear_mappings(struct amdgpu_device *adev,
>   		    tmp->last = eaddr;
>   
>   		tmp->bo_va = NULL;
> +		spin_lock(&vm->status_lock);
>   		list_add(&tmp->list, &vm->freed);
> +		spin_unlock(&vm->status_lock);
>   		trace_amdgpu_vm_bo_unmap(NULL, tmp);
>   	}
>   
> @@ -1788,7 +1811,9 @@ void amdgpu_vm_bo_del(struct amdgpu_device *adev,
>   		amdgpu_vm_it_remove(mapping, &vm->va);
>   		mapping->bo_va = NULL;
>   		trace_amdgpu_vm_bo_unmap(bo_va, mapping);
> +		spin_lock(&vm->status_lock);
>   		list_add(&mapping->list, &vm->freed);
> +		spin_unlock(&vm->status_lock);
>   	}
>   	list_for_each_entry_safe(mapping, next, &bo_va->invalids, list) {
>   		list_del(&mapping->list);