Re: [PATCH v2 2/8] media: videobuf2: Make bufs array dynamic allocated

[Hans Verkuil]

On 24/03/2023 09:48, Laurent Pinchart wrote:
> On Fri, Mar 24, 2023 at 09:31:35AM +0100, Hans Verkuil wrote:
>> On 24/03/2023 09:11, Benjamin Gaignard wrote:
>>>
>>> Le 24/03/2023 à 06:01, Dan Carpenter a écrit :
>>>> Hi Benjamin,
>>>>
>>>> https://git-scm.com/docs/git-format-patch#_base_tree_information]
>>>>
>>>> url:    https://github.com/intel-lab-lkp/linux/commits/Benjamin-Gaignard/media-videobuf2-Access-vb2_queue-bufs-array-through-helper-functions/20230321-183154
>>>> base:   git://linuxtv.org/media_tree.git master
>>>> patch link:    https://lore.kernel.org/r/20230321102855.346732-3-benjamin.gaignard%40collabora.com
>>>> patch subject: [PATCH v2 2/8] media: videobuf2: Make bufs array dynamic allocated
>>>> config: arm64-randconfig-m041-20230319 (https://download.01.org/0day-ci/archive/20230324/202303240148.lKRnUqW9-lkp@xxxxxxxxx/config)
>>>> compiler: aarch64-linux-gcc (GCC) 12.1.0
>>>>
>>>> If you fix the issue, kindly add following tag where applicable
>>>> | Reported-by: kernel test robot <lkp@xxxxxxxxx>
>>>> | Reported-by: Dan Carpenter <error27@xxxxxxxxx>
>>>> | Link: https://lore.kernel.org/r/202303240148.lKRnUqW9-lkp@xxxxxxxxx/
>>>>
>>>> smatch warnings:
>>>> include/media/videobuf2-core.h:1272 vb2_queue_add_buffer() warn: sleeping in atomic context
>>>> drivers/media/common/videobuf2/videobuf2-core.c:2456 vb2_core_queue_init() warn: Please consider using kcalloc instead of kmalloc_array
>>>>
>>>> vim +1272 include/media/videobuf2-core.h
>>>>
>>>> 625d46c1c1fe8e Benjamin Gaignard 2023-03-21  1263  static inline bool vb2_queue_add_buffer(struct vb2_queue *q, struct vb2_buffer *vb)
>>>> 625d46c1c1fe8e Benjamin Gaignard 2023-03-21  1264  {
>>>> 487d3f14d12ecf Benjamin Gaignard 2023-03-21  1265      bool ret = false;
>>>> 487d3f14d12ecf Benjamin Gaignard 2023-03-21  1266
>>>> 487d3f14d12ecf Benjamin Gaignard 2023-03-21  1267      spin_lock(&q->bufs_lock);
>>>>                                                          ^^^^^^^^^^^^^^^^^^^^^^^
>>>> Holding a spin lock.
>>>>
>>>> 487d3f14d12ecf Benjamin Gaignard 2023-03-21  1268
>>>> 487d3f14d12ecf Benjamin Gaignard 2023-03-21  1269      if (vb->index >= q->max_num_bufs) {
>>>> 487d3f14d12ecf Benjamin Gaignard 2023-03-21  1270          struct vb2_buffer **tmp;
>>>> 487d3f14d12ecf Benjamin Gaignard 2023-03-21  1271
>>>> 487d3f14d12ecf Benjamin Gaignard 2023-03-21 @1272          tmp = krealloc_array(q->bufs, q->max_num_bufs * 2, sizeof(*q->bufs), GFP_KERNEL);
>>>>                                                                                                                                       ^^^^^^^^^^
>>>> Sleeping allocation.  GFP_ATOMIC?  Or is there a way to move the
>>>> allocation outside the lock?
>>>
>>> I will add GFP_ATOMIC flag in next version.
>>
>> No need. Instead, don't use realloc here, just allocate a new array, copy over all
>> the data from the old, and then switch q->bufs with the spinlock held. Then you
>> can free the old one.
>>
>> It's only when you update q->bufs that you need the lock.
> 
> The copy also needs to be protected by the lock.

I suspect that that is not needed, since you shouldn't be able to add buffers here
since a mutex should be held at this time.

That said, it's something that Benjamin needs to analyze.

Regards,

	Hans

> 
>>>> 487d3f14d12ecf Benjamin Gaignard 2023-03-21  1273          if (!tmp)
>>>> 487d3f14d12ecf Benjamin Gaignard 2023-03-21  1274              goto realloc_failed;
>>>> 487d3f14d12ecf Benjamin Gaignard 2023-03-21  1275
>>>> 487d3f14d12ecf Benjamin Gaignard 2023-03-21  1276          q->max_num_bufs *= 2;
>>>> 487d3f14d12ecf Benjamin Gaignard 2023-03-21  1277          q->bufs = tmp;
>>>> 487d3f14d12ecf Benjamin Gaignard 2023-03-21  1278      }
>>>> 487d3f14d12ecf Benjamin Gaignard 2023-03-21  1279
>>>> 487d3f14d12ecf Benjamin Gaignard 2023-03-21  1280      if (vb->index < q->max_num_bufs) {
>>>> 625d46c1c1fe8e Benjamin Gaignard 2023-03-21  1281          q->bufs[vb->index] = vb;
>>>> 487d3f14d12ecf Benjamin Gaignard 2023-03-21  1282          ret = true;
>>>> 625d46c1c1fe8e Benjamin Gaignard 2023-03-21  1283      }
>>>> 625d46c1c1fe8e Benjamin Gaignard 2023-03-21  1284
>>>> 487d3f14d12ecf Benjamin Gaignard 2023-03-21  1285  realloc_failed:
>>>> 487d3f14d12ecf Benjamin Gaignard 2023-03-21  1286      spin_unlock(&q->bufs_lock);
>>>> 487d3f14d12ecf Benjamin Gaignard 2023-03-21  1287
>>>> 487d3f14d12ecf Benjamin Gaignard 2023-03-21  1288      return ret;
>>>> 625d46c1c1fe8e Benjamin Gaignard 2023-03-21  1289  }
>