Re: [PATCH] drm/panthor: fix for dma-fence safe access rules

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

On Wed,  3 Dec 2025 17:50:34 -0800
Chia-I Wu <olvaffe@gmail.com> wrote:

> Commit 506aa8b02a8d6 ("dma-fence: Add safe access helpers and document
> the rules") details the dma-fence safe access rules. The most common
> culprit is that drm_sched_fence_get_timeline_name may race with
> group_free_queue.
> 
> Fixes: d2624d90a0b77 ("drm/panthor: assign unique names to queues")
> Signed-off-by: Chia-I Wu <olvaffe@gmail.com>

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

> ---
>  drivers/gpu/drm/panthor/panthor_sched.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/drivers/gpu/drm/panthor/panthor_sched.c b/drivers/gpu/drm/panthor/panthor_sched.c
> index 33b9ef537e359..a8b1347e4da71 100644
> --- a/drivers/gpu/drm/panthor/panthor_sched.c
> +++ b/drivers/gpu/drm/panthor/panthor_sched.c
> @@ -23,6 +23,7 @@
>  #include <linux/module.h>
>  #include <linux/platform_device.h>
>  #include <linux/pm_runtime.h>
> +#include <linux/rcupdate.h>
>  
>  #include "panthor_devfreq.h"
>  #include "panthor_device.h"
> @@ -923,6 +924,9 @@ static void group_release_work(struct work_struct *work)
>  						   release_work);
>  	u32 i;
>  
> +	/* dma-fences may still be accessing group->queues under rcu lock. */
> +	synchronize_rcu();
> +
>  	for (i = 0; i < group->queue_count; i++)
>  		group_free_queue(group, group->queues[i]);
>