Re: [PATCH v3 1/2] drm/panfrost: Fix dma_resv deadlock at drm object pin time

[Boris Brezillon <boris.brezillon@collabora.com>]

On Wed,  1 May 2024 07:55:59 +0100
Adrián Larumbe <adrian.larumbe@collabora.com> wrote:

> When Panfrost must pin an object that is being prepared a dma-buf
> attachment for on behalf of another driver, the core drm gem object pinning
> code already takes a lock on the object's dma reservation.
> 
> However, Panfrost GEM object's pinning callback would eventually try taking
> the lock on the same dma reservation when delegating pinning of the object
> onto the shmem subsystem, which led to a deadlock.
> 
> This can be shown by enabling CONFIG_DEBUG_WW_MUTEX_SLOWPATH, which throws
> the following recursive locking situation:
> 
> weston/3440 is trying to acquire lock:
> ffff000000e235a0 (reservation_ww_class_mutex){+.+.}-{3:3}, at: drm_gem_shmem_pin+0x34/0xb8 [drm_shmem_helper]
> but task is already holding lock:
> ffff000000e235a0 (reservation_ww_class_mutex){+.+.}-{3:3}, at: drm_gem_pin+0x2c/0x80 [drm]
> 
> Fix it by assuming the object's reservation had already been locked by the
> time we reach panfrost_gem_pin.

You should probably mention that drm_gem_shmem_object_pin() assumes the
lock to be held, thus explaining the drm_gem_shmem_pin() ->
drm_gem_shmem_object_pin() transition.

Oh, and the commit message refers explicitly to Panfrost in a few places
even though you're fixing Lima as well.

> 
> Do the same thing for the Lima driver, as it most likely suffers from the
> same issue.
> 
> Cc: Thomas Zimmermann <tzimmermann@suse.de>
> Cc: Dmitry Osipenko <dmitry.osipenko@collabora.com>
> Cc: Boris Brezillon <boris.brezillon@collabora.com>
> Cc: Steven Price <steven.price@arm.com>
> Fixes: a78027847226 ("drm/gem: Acquire reservation lock in drm_gem_{pin/unpin}()")
> Signed-off-by: Adrián Larumbe <adrian.larumbe@collabora.com>

With the commit message adjusted as suggested,

Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>

> ---
>  drivers/gpu/drm/lima/lima_gem.c         | 2 +-
>  drivers/gpu/drm/panfrost/panfrost_gem.c | 2 +-
>  2 files changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/gpu/drm/lima/lima_gem.c b/drivers/gpu/drm/lima/lima_gem.c
> index 7ea244d876ca..c4e0f9faaa47 100644
> --- a/drivers/gpu/drm/lima/lima_gem.c
> +++ b/drivers/gpu/drm/lima/lima_gem.c
> @@ -185,7 +185,7 @@ static int lima_gem_pin(struct drm_gem_object *obj)
>  	if (bo->heap_size)
>  		return -EINVAL;
>  
> -	return drm_gem_shmem_pin(&bo->base);
> +	return drm_gem_shmem_object_pin(obj);
>  }
>  
>  static int lima_gem_vmap(struct drm_gem_object *obj, struct iosys_map *map)
> diff --git a/drivers/gpu/drm/panfrost/panfrost_gem.c b/drivers/gpu/drm/panfrost/panfrost_gem.c
> index d47b40b82b0b..f268bd5c2884 100644
> --- a/drivers/gpu/drm/panfrost/panfrost_gem.c
> +++ b/drivers/gpu/drm/panfrost/panfrost_gem.c
> @@ -192,7 +192,7 @@ static int panfrost_gem_pin(struct drm_gem_object *obj)
>  	if (bo->is_heap)
>  		return -EINVAL;
>  
> -	return drm_gem_shmem_pin(&bo->base);
> +	return drm_gem_shmem_object_pin(obj);
>  }
>  
>  static enum drm_gem_object_status panfrost_gem_status(struct drm_gem_object *obj)