linux-kernel - Re: [PATCH v3 2/3] media: uvcvideo: Do not halt the device after disconnect

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Message-ID: <20231122074742.GB1465745@google.com>
Date:   Wed, 22 Nov 2023 16:47:42 +0900
From:   Sergey Senozhatsky <senozhatsky@...omium.org>
To:     Ricardo Ribalda <ribalda@...omium.org>
Cc:     Mauro Carvalho Chehab <mchehab@...nel.org>,
        Guenter Roeck <linux@...ck-us.net>,
        Tomasz Figa <tfiga@...omium.org>,
        Laurent Pinchart <laurent.pinchart@...asonboard.com>,
        Alan Stern <stern@...land.harvard.edu>,
        Hans Verkuil <hverkuil-cisco@...all.nl>,
        linux-media@...r.kernel.org, linux-kernel@...r.kernel.org,
        Sean Paul <seanpaul@...omium.org>,
        Sakari Ailus <sakari.ailus@...ux.intel.com>
Subject: Re: [PATCH v3 2/3] media: uvcvideo: Do not halt the device after
 disconnect

On (23/11/21 19:53), Ricardo Ribalda wrote:
> diff --git a/drivers/media/usb/uvc/uvc_driver.c b/drivers/media/usb/uvc/uvc_driver.c
> index 08fcd2ffa727..413c32867617 100644
> --- a/drivers/media/usb/uvc/uvc_driver.c
> +++ b/drivers/media/usb/uvc/uvc_driver.c
> @@ -2257,6 +2257,8 @@ static void uvc_disconnect(struct usb_interface *intf)
>  		return;
>  
>  	uvc_unregister_video(dev);
> +	/* Barrier needed to synchronize with uvc_video_stop_streaming(). */
> +	smp_store_release(&dev->disconnected, true);
>  	kref_put(&dev->ref, uvc_delete);
>  }

[..]

> +void uvc_video_stop_streaming(struct uvc_streaming *stream)
> +{
> +	uvc_video_stop_transfer(stream, 1);
> +
> +	/*
> +	 * Barrier needed to synchronize with uvc_disconnect().
> +	 * We cannot call usb_* functions on a disconnected USB device.
> +	 */
> +	if (!smp_load_acquire(&stream->dev->disconnected))
> +		uvc_video_halt(stream);
> +
>  	uvc_video_clock_cleanup(stream);
>  }

Can the following happen?

CPU0                                            CPU1
 uvc_disconnect()
						uvc_video_stop_streaming()
 usb_set_intfdata()
 uvc_unregister_video()

						if (!smp_load(&dev->disconnected))
							uvc_video_halt()

 smp_store_release(&dev->disconnected, true);

 kref_put(&dev->ref, uvc_delete);