On Fri, Mar 24, 2023 at 09:56:34AM +0100, Benjamin Gaignard wrote:

Le 24/03/2023 à 09:52, Hans Verkuil a écrit :

quoted

On 24/03/2023 09:48, Laurent Pinchart wrote:

quoted

On Fri, Mar 24, 2023 at 09:31:35AM +0100, Hans Verkuil wrote:

quoted

On 24/03/2023 09:11, Benjamin Gaignard wrote:

quoted

Le 24/03/2023 à 06:01, Dan Carpenter a écrit :

quoted

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@intel.com/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 [off-list ref]
| Reported-by: Dan Carpenter [off-list ref]
| Link: https://lore.kernel.org/r/202303240148.lKRnUqW9-lkp@intel.com/ (local)

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.

I spent some time looking through the call sites of vb2_get_buffer() and
how those can be called and it turned out to be a massive list of code
paths, including a lot of calls originating from codec drivers calling
vb2_find_buffer() in random contexts (possibly even interrupt). So a
spinlock protecting the array pointer makes sense indeed.

Does using GFP_ATOMIC is problematic ?

Yes, because the ability to reclaim memory is drastically limited and
the allocation is more likely to fail (as in: it's actually possible).
(And generally the time with interrupts disabled should be minimized to
keep system latency reasonable.)

Best regards,
Tomasz

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