Re: [PATCH] v4l2-dev: Fix race condition on __video_register_device

[Hans Verkuil <hverkuil@xs4all.nl>]

On Tue 13 August 2013 17:04:06 Ricardo Ribalda Delgado wrote:
> When 2 devices are registered at the same time, in Part 2 both will get
> the same minor number, making Part6 crash, because it cannot create a
> create a device with a duplicated minor:

You would expect that such core code would be bug free by now, but this is
indeed a race condition.

Acked-by: Hans Verkuil <hans.verkuil@cisco.com>

Regards,

	Hans

> 
> [    7.157648] ------------[ cut here ]------------
> [    7.157666] WARNING: at fs/sysfs/dir.c:530 sysfs_add_one+0xbd/0xe0()
> [    7.157669] sysfs: cannot create duplicate filename '/dev/char/81:1'
> [    7.157672] Modules linked in: qtec_xform(+) qt5023_video(+) videobuf2_vmalloc videobuf2_dma_sg videobuf2_memops videobuf2_core gpio_xilinx(+) qtec_white qtec_cmosis(+) qtec_pcie qt5023
> [    7.157694] CPU: 0 PID: 120 Comm: systemd-udevd Not tainted 3.10.0-qtec-standard #8
> [    7.157698] Hardware name: QTechnology QT5022/QT5022, BIOS PM_2.1.0.309 X64 05/23/2013
> [    7.157702]  0000000000000009 ffff8801788358e8 ffffffff8176c487 ffff880178835928
> [    7.157707]  ffffffff8106f6f0 ffff880175759770 00000000ffffffef ffff880175759930
> [    7.157712]  ffff8801788359e8 ffff880178bff000 ffff880175759930 ffff880178835988
> [    7.157718] Call Trace:
> [    7.157728]  [<ffffffff8176c487>] dump_stack+0x19/0x1b
> [    7.157735]  [<ffffffff8106f6f0>] warn_slowpath_common+0x70/0xa0
> [    7.157740]  [<ffffffff8106f7d6>] warn_slowpath_fmt+0x46/0x50
> [    7.157746]  [<ffffffff81324b35>] ? strlcat+0x65/0x90
> [    7.157750]  [<ffffffff811d516d>] sysfs_add_one+0xbd/0xe0
> [    7.157755]  [<ffffffff811d5c6b>] sysfs_do_create_link_sd+0xdb/0x200
> [    7.157760]  [<ffffffff811d5db1>] sysfs_create_link+0x21/0x40
> [    7.157765]  [<ffffffff813e277b>] device_add+0x21b/0x6d0
> [    7.157772]  [<ffffffff813f2985>] ? pm_runtime_init+0xe5/0xf0
> [    7.157776]  [<ffffffff813e2c4e>] device_register+0x1e/0x30
> [    7.157782]  [<ffffffff8153e8c3>] __video_register_device+0x313/0x610
> [    7.157791]  [<ffffffffa00957c5>] qtec_xform_probe+0x465/0x7a4 [qtec_xform]
> [    7.157797]  [<ffffffff813e7b13>] platform_drv_probe+0x43/0x80
> [    7.157802]  [<ffffffff813e531a>] ? driver_sysfs_add+0x7a/0xb0
> [    7.157807]  [<ffffffff813e584b>] driver_probe_device+0x8b/0x3a0
> [    7.157812]  [<ffffffff813e5c0b>] __driver_attach+0xab/0xb0
> [    7.157816]  [<ffffffff813e5b60>] ? driver_probe_device+0x3a0/0x3a0
> [    7.157820]  [<ffffffff813e37fd>] bus_for_each_dev+0x5d/0xa0
> [    7.157825]  [<ffffffff813e529e>] driver_attach+0x1e/0x20
> [    7.157829]  [<ffffffff813e4d3e>] bus_add_driver+0x10e/0x280
> [    7.157833]  [<ffffffffa0099000>] ? 0xffffffffa0098fff
> [    7.157837]  [<ffffffffa0099000>] ? 0xffffffffa0098fff
> [    7.157842]  [<ffffffff813e6317>] driver_register+0x77/0x170
> [    7.157848]  [<ffffffff81151bcc>] ? __vunmap+0x9c/0x110
> [    7.157852]  [<ffffffffa0099000>] ? 0xffffffffa0098fff
> [    7.157857]  [<ffffffff813e7236>] platform_driver_register+0x46/0x50
> [    7.157863]  [<ffffffffa0099010>] qtec_xform_plat_driver_init+0x10/0x12 [qtec_xform]
> [    7.157869]  [<ffffffff810002ea>] do_one_initcall+0xea/0x1a0
> [    7.157875]  [<ffffffff810cf1f1>] load_module+0x1a91/0x2630
> [    7.157880]  [<ffffffff8133bee0>] ? ddebug_proc_show+0xe0/0xe0
> [    7.157887]  [<ffffffff817760f2>] ? page_fault+0x22/0x30
> [    7.157892]  [<ffffffff810cfe7a>] SyS_init_module+0xea/0x140
> [    7.157898]  [<ffffffff8177e5b9>] tracesys+0xd0/0xd5
> [    7.157902] ---[ end trace 660cc3a65a4bf01b ]---
> [    7.157939] __video_register_device: device_register failed
> 
> Signed-off-by: Ricardo Ribalda Delgado <ricardo.ribalda@gmail.com>
> ---
>  drivers/media/v4l2-core/v4l2-dev.c |    5 ++---
>  1 file changed, 2 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/media/v4l2-core/v4l2-dev.c b/drivers/media/v4l2-core/v4l2-dev.c
> index 5923c5d..3f75f99 100644
> --- a/drivers/media/v4l2-core/v4l2-dev.c
> +++ b/drivers/media/v4l2-core/v4l2-dev.c
> @@ -873,6 +873,7 @@ int __video_register_device(struct video_device *vdev, int type, int nr,
>  
>  	/* Should not happen since we thought this minor was free */
>  	WARN_ON(video_device[vdev->minor] != NULL);
> +	video_device[vdev->minor] = vdev;
>  	vdev->index = get_index(vdev);
>  	mutex_unlock(&videodev_lock);
>  
> @@ -936,9 +937,6 @@ int __video_register_device(struct video_device *vdev, int type, int nr,
>  #endif
>  	/* Part 6: Activate this minor. The char device can now be used. */
>  	set_bit(V4L2_FL_REGISTERED, &vdev->flags);
> -	mutex_lock(&videodev_lock);
> -	video_device[vdev->minor] = vdev;
> -	mutex_unlock(&videodev_lock);
>  
>  	return 0;
>  
> @@ -946,6 +944,7 @@ cleanup:
>  	mutex_lock(&videodev_lock);
>  	if (vdev->cdev)
>  		cdev_del(vdev->cdev);
> +	video_device[vdev->minor] = NULL;
>  	devnode_clear(vdev);
>  	mutex_unlock(&videodev_lock);
>  	/* Mark this video device as never having been registered. */
>