[PATCH v2] drm/panfrost: Fix dma_resv deadlock at drm object pin time

[PATCH v2] drm/panfrost: Fix dma_resv deadlock at drm object pin time

[Adrián Larumbe]

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.

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}()")
Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Signed-off-by: Adrián Larumbe <adrian.larumbe@collabora.com>
---
 drivers/gpu/drm/lima/lima_gem.c         | 9 +++++++--
 drivers/gpu/drm/panfrost/panfrost_gem.c | 8 +++++++-
 2 files changed, 14 insertions(+), 3 deletions(-)

diff --git a/drivers/gpu/drm/lima/lima_gem.c b/drivers/gpu/drm/lima/lima_gem.c
index 7ea244d876ca..8a5bcf498ef6 100644
--- a/drivers/gpu/drm/lima/lima_gem.c
+++ b/drivers/gpu/drm/lima/lima_gem.c
@@ -184,8 +184,13 @@ static int lima_gem_pin(struct drm_gem_object *obj)
 
 	if (bo->heap_size)
 		return -EINVAL;
-
-	return drm_gem_shmem_pin(&bo->base);
+	/*
+	 * Pinning can only happen in response to a prime attachment request
+	 * from another driver, but dma reservation locking is already being
+	 * handled by drm_gem_pin
+	 */
+	drm_WARN_ON(obj->dev, obj->import_attach);
+	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..e3fbcb020617 100644
--- a/drivers/gpu/drm/panfrost/panfrost_gem.c
+++ b/drivers/gpu/drm/panfrost/panfrost_gem.c
@@ -192,7 +192,13 @@ static int panfrost_gem_pin(struct drm_gem_object *obj)
 	if (bo->is_heap)
 		return -EINVAL;
 
-	return drm_gem_shmem_pin(&bo->base);
+	/*
+	 * Pinning can only happen in response to a prime attachment request
+	 * from another driver, but dma reservation locking is already being
+	 * handled by drm_gem_pin
+	 */
+	drm_WARN_ON(obj->dev, obj->import_attach);
+	return drm_gem_shmem_object_pin(obj);
 }
 
 static enum drm_gem_object_status panfrost_gem_status(struct drm_gem_object *obj)
-- 
2.44.0

Re: [PATCH v2] drm/panfrost: Fix dma_resv deadlock at drm object pin time

[Boris Brezillon]

On Tue, 23 Apr 2024 23:46:23 +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.
> 
> 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}()")
> Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
> Signed-off-by: Adrián Larumbe <adrian.larumbe@collabora.com>
> ---
>  drivers/gpu/drm/lima/lima_gem.c         | 9 +++++++--
>  drivers/gpu/drm/panfrost/panfrost_gem.c | 8 +++++++-
>  2 files changed, 14 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/gpu/drm/lima/lima_gem.c b/drivers/gpu/drm/lima/lima_gem.c
> index 7ea244d876ca..8a5bcf498ef6 100644
> --- a/drivers/gpu/drm/lima/lima_gem.c
> +++ b/drivers/gpu/drm/lima/lima_gem.c
> @@ -184,8 +184,13 @@ static int lima_gem_pin(struct drm_gem_object *obj)
>  
>  	if (bo->heap_size)
>  		return -EINVAL;
> -
> -	return drm_gem_shmem_pin(&bo->base);
> +	/*
> +	 * Pinning can only happen in response to a prime attachment request
> +	 * from another driver, but dma reservation locking is already being
> +	 * handled by drm_gem_pin

This comment looks a bit weird now that you call a function that
doesn't have the _locked suffix. I'd be tempted to drop it, or clarify
the fact drm_gem_shmem_object_pin() expects the resv lock to be held.

> +	 */
> +	drm_WARN_ON(obj->dev, obj->import_attach);

Should this be moved to drm_gem_shmem_[un]pin_locked() instead of being
duplicated in all overloads of ->[un]pin()?

> +	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..e3fbcb020617 100644
> --- a/drivers/gpu/drm/panfrost/panfrost_gem.c
> +++ b/drivers/gpu/drm/panfrost/panfrost_gem.c
> @@ -192,7 +192,13 @@ static int panfrost_gem_pin(struct drm_gem_object *obj)
>  	if (bo->is_heap)
>  		return -EINVAL;
>  
> -	return drm_gem_shmem_pin(&bo->base);
> +	/*
> +	 * Pinning can only happen in response to a prime attachment request
> +	 * from another driver, but dma reservation locking is already being
> +	 * handled by drm_gem_pin
> +	 */
> +	drm_WARN_ON(obj->dev, obj->import_attach);
> +	return drm_gem_shmem_object_pin(obj);
>  }
>  
>  static enum drm_gem_object_status panfrost_gem_status(struct drm_gem_object *obj)