linux-kernel - Re: [PATCH v2] drm/panthor: fix for dma-fence safe access rules

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Message-ID: <42a3d2dc-1e1a-49ef-b3e7-c2fe604ca09a@arm.com>
Date: Wed, 17 Dec 2025 15:55:42 +0000
From: Steven Price <steven.price@....com>
To: Chia-I Wu <olvaffe@...il.com>,
 Boris Brezillon <boris.brezillon@...labora.com>
Cc: Liviu Dudau <liviu.dudau@....com>,
 Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
 Maxime Ripard <mripard@...nel.org>, Thomas Zimmermann <tzimmermann@...e.de>,
 David Airlie <airlied@...il.com>, Simona Vetter <simona@...ll.ch>,
 Grant Likely <grant.likely@...aro.org>, Heiko Stuebner <heiko@...ech.de>,
 dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
 tvrtko.ursulin@...lia.com
Subject: Re: [PATCH v2] drm/panthor: fix for dma-fence safe access rules

On 05/12/2025 18:45, Chia-I Wu wrote:
> On Fri, Dec 5, 2025 at 4:43 AM Boris Brezillon
> <boris.brezillon@...labora.com> wrote:
>>
>> On Thu,  4 Dec 2025 09:45:45 -0800
>> Chia-I Wu <olvaffe@...il.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.
>>>
>>> Signed-off-by: Chia-I Wu <olvaffe@...il.com>
>>> Reviewed-by: Boris Brezillon <boris.brezillon@...labora.com>
>>> Reviewed-by: Liviu Dudau <liviu.dudau@....com>
>>> Reviewed-by: Steven Price <steven.price@....com>
>>>
>>> ---
>>> v2: collect R-bs and drop misleading Fixes: tag
>>
>> Should we at least
>>
>> Cc: stable@...r.kernel.org
>>
>> so this gets considered for backporting?
> This can be considered a follow-up to commit 506aa8b02a8d6. I think
> 
>   Cc: stable@...r.kernel.org # v6.17+
> 
> is appropriate. (Not sure if I should send v3 or leave it to
> maintainers, because I was once told to never add the line and always
> leave it to maintainers).
> 
> To help make the decision, commit 506aa8b02a8d6 itself narrowed the
> race window greatly. This follow-up closes the remaining case where
> panthor signals the fence and destroys the group while the consumer
> holds the rcu read lock.
> 

I've pushed this to drm-misc-next with the Cc tag.

Thanks,
Steve

>>
>>> ---
>>>  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]);
>>>
>>