[PATCH] drm/amdgpu: Fix user queue deadlock by reordering mutex locking

[Christian König]

On 5/9/25 11:21, Jesse.Zhang wrote:
> This resolves a deadlock between user queue management and GPU reset
> paths by enforcing consistent lock ordering.
> 
> The deadlock occurred when:
> 
> 1. Process exit path (amdgpu_userq_mgr_fini) would:
>    - Take uqm->userq_mutex
>    - Then try to take adev->userq_mutex for list operations
> 
> 2. GPU reset path (amdgpu_userq_pre_reset) would:
>    - Take adev->userq_mutex first (for list traversal)
>    - Then take uqm->userq_mutex
> 
> The solution establishes a strict top-down locking order:
> 1. Always take adev->userq_mutex before any uqm->userq_mutex
> 2. Maintain this order consistently across all code paths
> 
> Changes made:
> - Reordered locking in amdgpu_userq_mgr_fini() to take device lock first
> - Kept existing proper order in amdgpu_userq_pre_reset()
> - Simplified the fini flow by removing redundant operations
> 
> This prevents circular dependencies while maintaining thread safety
> during both normal operation and GPU reset scenarios.
> 
> Fixes: e274db8c826 ("drm/amdgpu: store userq_managers in a list in adev")
> 
> Signed-off-by: Jesse Zhang <Jesse.Zhang at amd.com>

Reviewed-by: Christian König <christian.koenig at amd.com>

> ---
>  drivers/gpu/drm/amd/amdgpu/amdgpu_userq.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_userq.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_userq.c
> index 1c0ddec06280..0f1cb6bc63db 100644
> --- a/drivers/gpu/drm/amd/amdgpu/amdgpu_userq.c
> +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_userq.c
> @@ -879,22 +879,23 @@ void amdgpu_userq_mgr_fini(struct amdgpu_userq_mgr *userq_mgr)
>  
>  	cancel_delayed_work_sync(&userq_mgr->resume_work);
>  
> +	mutex_lock(&adev->userq_mutex);
>  	mutex_lock(&userq_mgr->userq_mutex);
>  	idr_for_each_entry(&userq_mgr->userq_idr, queue, queue_id) {
>  		amdgpu_userq_wait_for_last_fence(userq_mgr, queue);
>  		amdgpu_userq_unmap_helper(userq_mgr, queue);
>  		amdgpu_userq_cleanup(userq_mgr, queue, queue_id);
>  	}
> -	mutex_lock(&adev->userq_mutex);
> +
>  	list_for_each_entry_safe(uqm, tmp, &adev->userq_mgr_list, list) {
>  		if (uqm == userq_mgr) {
>  			list_del(&uqm->list);
>  			break;
>  		}
>  	}
> -	mutex_unlock(&adev->userq_mutex);
>  	idr_destroy(&userq_mgr->userq_idr);
>  	mutex_unlock(&userq_mgr->userq_mutex);
> +	mutex_unlock(&adev->userq_mutex);
>  	mutex_destroy(&userq_mgr->userq_mutex);
>  }
>