Received: by 2002:a05:6a10:a0d1:0:0:0:0 with SMTP id j17csp1756873pxa; Sun, 16 Aug 2020 10:00:10 -0700 (PDT) X-Google-Smtp-Source: ABdhPJxEjdXoBUrlyOBmCFFk4WpfmN3Hdp8AlE5RudfuNaBA/eYYioZkXp5qYqAW6l4dqnvme/nP X-Received: by 2002:a17:906:7d90:: with SMTP id v16mr11363086ejo.27.1597597210537; Sun, 16 Aug 2020 10:00:10 -0700 (PDT) ARC-Seal: i=1; a=rsa-sha256; t=1597597210; cv=none; d=google.com; s=arc-20160816; b=PSiq64WlPXozUC8CrCYvqHyBFM1fvHcMgF4xrXjObgmwN3eGmdSfUVje1hGfbTgMHi c1g1bNKjaW1Ip+q0oA5XXGO3IrLDOV7S7DaLGF57UzdhT28CUv5Kokhie3nQvRqwhlUR 0kFcb4p74tRxzUU3hkQ85Vb91XV8a9Cd+dDQAiTwd55NA9zoBTqzlBUkIX4VDnk6Mhl3 zqFJHVpGA3uiygOcTuPFs/ZhzfmyQhoF9uysY3Et+87Efv8v2cvH5ZApBfNt5qK/MzwX fzw7/njf4kyHeRFKczgS/qAGdNn6AAEdoyWGklwsCh/6crJTtUGNMYxrAmxtQP0mEB8L frDQ== ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=arc-20160816; h=list-id:precedence:sender:content-transfer-encoding :content-language:in-reply-to:mime-version:user-agent:date :message-id:autocrypt:from:references:cc:to:subject:dkim-signature; bh=ROKHDEP/OCDBcnDVvI/zczMDQypCsxjEouHK4qoGXdA=; b=REQ2kpSBLFWuqKvctGengtZc/cYBt9bZgVKLEgj+6tMDonKmgnkWU7L11bBHl/r+GT io0J26apEY/eAhkKd8JqtG9nWVKD97SkEqONt73ucf4NbzSq8yvFzOqlFhE7h12WFSQM TreSSZmzOWxTVqHfKAP3HIwrQNdcHEt/GMDNhDkl3ZA90151GRxa/DKbKaEjFNzNQrE9 6ryFK8sK7uRQC+SPlFEPS4niEsgDjQ2yznVE+2LLXvvIg3VMO7zul0a8LJmFmv1rhoR1 uaryj5lkPRvJlc+xzFlEuARdyWApbXRqvyccD+4Gg/vHSDL2gdZX+Z+kxf5eJJdHNfBv 8AJw== ARC-Authentication-Results: i=1; mx.google.com; dkim=fail header.i=@gmail.com header.s=20161025 header.b=eSQSHRkC; spf=pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) smtp.mailfrom=linux-kernel-owner@vger.kernel.org Return-Path: Received: from vger.kernel.org (vger.kernel.org. [23.128.96.18]) by mx.google.com with ESMTP id b6si10340271edq.179.2020.08.16.09.59.20; Sun, 16 Aug 2020 10:00:10 -0700 (PDT) Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18; Authentication-Results: mx.google.com; dkim=fail header.i=@gmail.com header.s=20161025 header.b=eSQSHRkC; spf=pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) smtp.mailfrom=linux-kernel-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728555AbgHPPyZ (ORCPT + 99 others); Sun, 16 Aug 2020 11:54:25 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:41974 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726407AbgHPPyW (ORCPT ); Sun, 16 Aug 2020 11:54:22 -0400 Received: from mail-pj1-x1044.google.com (mail-pj1-x1044.google.com [IPv6:2607:f8b0:4864:20::1044]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 909F9C061786; Sun, 16 Aug 2020 08:54:22 -0700 (PDT) Received: by mail-pj1-x1044.google.com with SMTP id f9so6491075pju.4; Sun, 16 Aug 2020 08:54:22 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20161025; h=sender:subject:to:cc:references:from:autocrypt:message-id:date :user-agent:mime-version:in-reply-to:content-language :content-transfer-encoding; bh=ROKHDEP/OCDBcnDVvI/zczMDQypCsxjEouHK4qoGXdA=; b=eSQSHRkCdBmD2PwlSRLk0/qF8bsnZMmQ03CM4u7DwJTCI/I69uKazhZfEsScaU6O3A Dl2RZVywGqdolrsBybq5W3jTxe4CuAhatKdgaPDFNwbAKu3x5WpdO7W/C1fMbUxApcCu HaHAlod3EzCJhWUw0fQhHBcUSftILsfMQdroFj4n+VyEQXUNx/ygEi6EoAxh63hwaeK2 rB9ZL8JMBU5GEXfjqpBtLLqDR50qTPEjkRaDXG3kP4IS5Y39l3WWGSHGRQOYCFdLR3wD z0kXpPnANO/UihQNKpseIM0G/dfHTB84pgFIQgHW64YI8r1paWPzHjdJb3sOlmct/pZk Ffzw== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:sender:subject:to:cc:references:from:autocrypt :message-id:date:user-agent:mime-version:in-reply-to :content-language:content-transfer-encoding; bh=ROKHDEP/OCDBcnDVvI/zczMDQypCsxjEouHK4qoGXdA=; b=H2jcpsiNkvoxRk5gTuUPXzq5wi3g50W6fIQ4jXvkYDzPsZdLNjQQVc0LLRMGbryLpW DxNx5aUok6kTNXa+W5CiPhmYHrMxjLuDYK2ibWqdhLoUBi7kPqcCc+MnlywsuTiCzbxH +1N0rGSyaMv35W2URiNvdXEwmOeLHr+v7mfp12Y4LANQGV7kGFQSxbjBaugqgdVarrY3 NFR0eINx3mXsQi/rsEIC936u4Szpu5a21VfIPcCxgKWUBUFjxMHi3xG1fsFhfH4AE1p3 9zdUWrs7RK2Ix2bXKWJ5+cLKg9jqYyKHBVM6lSPy4CXTgUh+sz021KnrYKiWRDoHskZZ usaA== X-Gm-Message-State: AOAM530oiAsGviZ10XEUh/ttyhWtXlsBKivqWRbhpG7xkwHCkZEJJwjg YAW4hG9xq/05Sa5n/osq0H8= X-Received: by 2002:a17:902:6b05:: with SMTP id o5mr8302571plk.173.1597593261278; Sun, 16 Aug 2020 08:54:21 -0700 (PDT) Received: from server.roeck-us.net ([2600:1700:e321:62f0:329c:23ff:fee3:9d7c]) by smtp.gmail.com with ESMTPSA id b185sm15733288pfg.71.2020.08.16.08.54.19 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Sun, 16 Aug 2020 08:54:20 -0700 (PDT) Subject: Re: Protecting uvcvideo againt USB device disconnect [Was: Re: Protecting usb_set_interface() against device removal] To: Laurent Pinchart Cc: linux-usb , Greg Kroah-Hartman , "linux-kernel@vger.kernel.org" , Alan Stern , linux-media@vger.kernel.org, linux-uvc-devel@lists.sourceforge.net, Hans Verkuil , Sakari Ailus References: <20200815020739.GB52242@rowland.harvard.edu> <20200816003315.GA13826@roeck-us.net> <20200816121816.GC32174@pendragon.ideasonboard.com> From: Guenter Roeck Autocrypt: addr=linux@roeck-us.net; keydata= xsFNBE6H1WcBEACu6jIcw5kZ5dGeJ7E7B2uweQR/4FGxH10/H1O1+ApmcQ9i87XdZQiB9cpN RYHA7RCEK2dh6dDccykQk3bC90xXMPg+O3R+C/SkwcnUak1UZaeK/SwQbq/t0tkMzYDRxfJ7 nyFiKxUehbNF3r9qlJgPqONwX5vJy4/GvDHdddSCxV41P/ejsZ8PykxyJs98UWhF54tGRWFl 7i1xvaDB9lN5WTLRKSO7wICuLiSz5WZHXMkyF4d+/O5ll7yz/o/JxK5vO/sduYDIlFTvBZDh gzaEtNf5tQjsjG4io8E0Yq0ViobLkS2RTNZT8ICq/Jmvl0SpbHRvYwa2DhNsK0YjHFQBB0FX IdhdUEzNefcNcYvqigJpdICoP2e4yJSyflHFO4dr0OrdnGLe1Zi/8Xo/2+M1dSSEt196rXaC kwu2KgIgmkRBb3cp2vIBBIIowU8W3qC1+w+RdMUrZxKGWJ3juwcgveJlzMpMZNyM1jobSXZ0 VHGMNJ3MwXlrEFPXaYJgibcg6brM6wGfX/LBvc/haWw4yO24lT5eitm4UBdIy9pKkKmHHh7s jfZJkB5fWKVdoCv/omy6UyH6ykLOPFugl+hVL2Prf8xrXuZe1CMS7ID9Lc8FaL1ROIN/W8Vk BIsJMaWOhks//7d92Uf3EArDlDShwR2+D+AMon8NULuLBHiEUQARAQABzTJHdWVudGVyIFJv ZWNrIChMaW51eCBhY2NvdW50KSA8bGludXhAcm9lY2stdXMubmV0PsLBgQQTAQIAKwIbAwYL CQgHAwIGFQgCCQoLBBYCAwECHgECF4ACGQEFAlVcphcFCRmg06EACgkQyx8mb86fmYFg0RAA nzXJzuPkLJaOmSIzPAqqnutACchT/meCOgMEpS5oLf6xn5ySZkl23OxuhpMZTVX+49c9pvBx hpvl5bCWFu5qC1jC2eWRYU+aZZE4sxMaAGeWenQJsiG9lP8wkfCJP3ockNu0ZXXAXwIbY1O1 c+l11zQkZw89zNgWgKobKzrDMBFOYtAh0pAInZ9TSn7oA4Ctejouo5wUugmk8MrDtUVXmEA9 7f9fgKYSwl/H7dfKKsS1bDOpyJlqhEAH94BHJdK/b1tzwJCFAXFhMlmlbYEk8kWjcxQgDWMu GAthQzSuAyhqyZwFcOlMCNbAcTSQawSo3B9yM9mHJne5RrAbVz4TWLnEaX8gA5xK3uCNCeyI sqYuzA4OzcMwnnTASvzsGZoYHTFP3DQwf2nzxD6yBGCfwNGIYfS0i8YN8XcBgEcDFMWpOQhT Pu3HeztMnF3HXrc0t7e5rDW9zCh3k2PA6D2NV4fews9KDFhLlTfCVzf0PS1dRVVWM+4jVl6l HRIAgWp+2/f8dx5vPc4Ycp4IsZN0l1h9uT7qm1KTwz+sSl1zOqKD/BpfGNZfLRRxrXthvvY8 BltcuZ4+PGFTcRkMytUbMDFMF9Cjd2W9dXD35PEtvj8wnEyzIos8bbgtLrGTv/SYhmPpahJA l8hPhYvmAvpOmusUUyB30StsHIU2LLccUPPOwU0ETofVZwEQALlLbQeBDTDbwQYrj0gbx3bq 7kpKABxN2MqeuqGr02DpS9883d/t7ontxasXoEz2GTioevvRmllJlPQERVxM8gQoNg22twF7 pB/zsrIjxkE9heE4wYfN1AyzT+AxgYN6f8hVQ7Nrc9XgZZe+8IkuW/Nf64KzNJXnSH4u6nJM J2+Dt274YoFcXR1nG76Q259mKwzbCukKbd6piL+VsT/qBrLhZe9Ivbjq5WMdkQKnP7gYKCAi pNVJC4enWfivZsYupMd9qn7Uv/oCZDYoBTdMSBUblaLMwlcjnPpOYK5rfHvC4opxl+P/Vzyz 6WC2TLkPtKvYvXmdsI6rnEI4Uucg0Au/Ulg7aqqKhzGPIbVaL+U0Wk82nz6hz+WP2ggTrY1w ZlPlRt8WM9w6WfLf2j+PuGklj37m+KvaOEfLsF1v464dSpy1tQVHhhp8LFTxh/6RWkRIR2uF I4v3Xu/k5D0LhaZHpQ4C+xKsQxpTGuYh2tnRaRL14YMW1dlI3HfeB2gj7Yc8XdHh9vkpPyuT nY/ZsFbnvBtiw7GchKKri2gDhRb2QNNDyBnQn5mRFw7CyuFclAksOdV/sdpQnYlYcRQWOUGY HhQ5eqTRZjm9z+qQe/T0HQpmiPTqQcIaG/edgKVTUjITfA7AJMKLQHgp04Vylb+G6jocnQQX JqvvP09whbqrABEBAAHCwWUEGAECAA8CGwwFAlVcpi8FCRmg08MACgkQyx8mb86fmYHNRQ/+ J0OZsBYP4leJvQF8lx9zif+v4ZY/6C9tTcUv/KNAE5leyrD4IKbnV4PnbrVhjq861it/zRQW cFpWQszZyWRwNPWUUz7ejmm9lAwPbr8xWT4qMSA43VKQ7ZCeTQJ4TC8kjqtcbw41SjkjrcTG wF52zFO4bOWyovVAPncvV9eGA/vtnd3xEZXQiSt91kBSqK28yjxAqK/c3G6i7IX2rg6pzgqh hiH3/1qM2M/LSuqAv0Rwrt/k+pZXE+B4Ud42hwmMr0TfhNxG+X7YKvjKC+SjPjqp0CaztQ0H nsDLSLElVROxCd9m8CAUuHplgmR3seYCOrT4jriMFBtKNPtj2EE4DNV4s7k0Zy+6iRQ8G8ng QjsSqYJx8iAR8JRB7Gm2rQOMv8lSRdjva++GT0VLXtHULdlzg8VjDnFZ3lfz5PWEOeIMk7Rj trjv82EZtrhLuLjHRCaG50OOm0hwPSk1J64R8O3HjSLdertmw7eyAYOo4RuWJguYMg5DRnBk WkRwrSuCn7UG+qVWZeKEsFKFOkynOs3pVbcbq1pxbhk3TRWCGRU5JolI4ohy/7JV1TVbjiDI HP/aVnm6NC8of26P40Pg8EdAhajZnHHjA7FrJXsy3cyIGqvg9os4rNkUWmrCfLLsZDHD8FnU mDW4+i+XlNFUPUYMrIKi9joBhu18ssf5i5Q= Message-ID: <9bb20ed7-b156-f6c2-4d25-6acac1a0021b@roeck-us.net> Date: Sun, 16 Aug 2020 08:54:18 -0700 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.10.0 MIME-Version: 1.0 In-Reply-To: <20200816121816.GC32174@pendragon.ideasonboard.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 8bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 8/16/20 5:18 AM, Laurent Pinchart wrote: > Hi Guenter, > > CC'ing Hans Verkuil and Sakari Ailus for the discussion about handling > file operations and disconnect in V4L2. > > On Sat, Aug 15, 2020 at 05:33:15PM -0700, Guenter Roeck wrote: >> + linux-uvc-devel@lists.sourceforge.net >> + linux-media@vger.kernel.org >> + laurent.pinchart@ideasonboard.com >> >> and changed subject >> >> On Fri, Aug 14, 2020 at 10:07:39PM -0400, Alan Stern wrote: >>> On Fri, Aug 14, 2020 at 04:07:03PM -0700, Guenter Roeck wrote: >>>> Hi all, >>>> >>>> over time, there have been a number of reports of crashes in usb_ifnum_to_if(), >>>> called from usb_hcd_alloc_bandwidth, which is in turn called from usb_set_interface(). >>>> Examples are [1] [2] [3]. A typical backtrace is: >>>> >>>> <3>[ 3489.445468] intel_sst_acpi 808622A8:00: sst: Busy wait failed, cant send this msg >>>> <6>[ 3490.507273] usb 1-4: USB disconnect, device number 3 >>>> <1>[ 3490.516670] BUG: unable to handle kernel NULL pointer dereference at 0000000000000000 >>>> <6>[ 3490.516680] PGD 0 P4D 0 >>>> <4>[ 3490.516687] Oops: 0000 [#1] PREEMPT SMP PTI >>>> <4>[ 3490.516693] CPU: 0 PID: 5633 Comm: V4L2CaptureThre Not tainted 4.19.113-08536-g5d29ca36db06 #1 >>>> <4>[ 3490.516696] Hardware name: GOOGLE Edgar, BIOS Google_Edgar.7287.167.156 03/25/2019 >>>> <4>[ 3490.516706] RIP: 0010:usb_ifnum_to_if+0x29/0x40 >>>> <4>[ 3490.516710] Code: ee 0f 1f 44 00 00 55 48 89 e5 48 8b 8f f8 03 00 00 48 85 c9 74 27 44 0f b6 41 04 4d 85 c0 74 1d 31 ff 48 8b 84 f9 98 00 00 00 <48> 8b 10 0f b6 52 02 39 f2 74 0a 48 ff c7 4c 39 c7 72 e5 31 c0 5d >>>> <4>[ 3490.516714] RSP: 0018:ffffa46f42a47a80 EFLAGS: 00010246 >>>> <4>[ 3490.516718] RAX: 0000000000000000 RBX: 0000000000000000 RCX: ffff904a396c9000 >>>> <4>[ 3490.516721] RDX: ffff904a39641320 RSI: 0000000000000001 RDI: 0000000000000000 >>>> <4>[ 3490.516724] RBP: ffffa46f42a47a80 R08: 0000000000000002 R09: 0000000000000000 >>>> <4>[ 3490.516727] R10: 0000000000009975 R11: 0000000000000009 R12: 0000000000000000 >>>> <4>[ 3490.516731] R13: ffff904a396b3800 R14: ffff904a39e88000 R15: 0000000000000000 >>>> <4>[ 3490.516735] FS: 00007f396448e700(0000) GS:ffff904a3ba00000(0000) knlGS:0000000000000000 >>>> <4>[ 3490.516738] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 >>>> <4>[ 3490.516742] CR2: 0000000000000000 CR3: 000000016cb46000 CR4: 00000000001006f0 >>>> <4>[ 3490.516745] Call Trace: >>>> <4>[ 3490.516756] usb_hcd_alloc_bandwidth+0x1ee/0x30f >>>> <4>[ 3490.516762] usb_set_interface+0x1a3/0x2b7 >>>> <4>[ 3490.516773] uvc_video_start_transfer+0x29b/0x4b8 [uvcvideo] >>>> <4>[ 3490.516781] uvc_video_start_streaming+0x91/0xdd [uvcvideo] >>>> <4>[ 3490.516787] uvc_start_streaming+0x28/0x5d [uvcvideo] >>>> <4>[ 3490.516795] vb2_start_streaming+0x61/0x143 [videobuf2_common] >>>> <4>[ 3490.516801] vb2_core_streamon+0xf7/0x10f [videobuf2_common] >>>> <4>[ 3490.516807] uvc_queue_streamon+0x2e/0x41 [uvcvideo] >>>> <4>[ 3490.516814] uvc_ioctl_streamon+0x42/0x5c [uvcvideo] >>>> <4>[ 3490.516820] __video_do_ioctl+0x33d/0x42a >>>> <4>[ 3490.516826] video_usercopy+0x34e/0x5ff >>>> <4>[ 3490.516831] ? video_ioctl2+0x16/0x16 >>>> <4>[ 3490.516837] v4l2_ioctl+0x46/0x53 >>>> <4>[ 3490.516843] do_vfs_ioctl+0x50a/0x76f >>>> <4>[ 3490.516848] ksys_ioctl+0x58/0x83 >>>> <4>[ 3490.516853] __x64_sys_ioctl+0x1a/0x1e >>>> <4>[ 3490.516858] do_syscall_64+0x54/0xde >>>> >>>> I have been able to reproduce the problem on a Chromebook by strategically placing >>>> msleep() calls into usb_set_interface() and usb_disable_device(). Ultimately, the >>>> problem boils down to lack of protection against device removal in usb_set_interface() >>>> [and/or possibly other callers of usb_ifnum_to_if()]. >>>> >>>> Sequence of events is roughly as follows: >>>> >>>> - usb_set_interface() is called and proceeds to some point, possibly to >>>> mutex_lock(hcd->bandwidth_mutex); >>>> - Device removal event is detected, and usb_disable_device() is called >>> >>> At this point all interface drivers get unbound (their disconnect >>> routines are called). >>> >>>> - usb_disable_device() starts removing actconfig data. It has removed >>>> and cleared dev->actconfig->interface[i], but not dev->actconfig >>>> - usb_set_interface() calls usb_hcd_alloc_bandwidth(), which calls >>>> usb_ifnum_to_if() >>>> - In usb_ifnum_to_if(), dev->actconfig is not NULL, but >>>> dev->actconfig->interface[i] is NULL >>>> - crash >>>> >>>> Question is what we can do about this. Checking if dev->state != USB_STATE_NOTATTACHED >>>> in usb_ifnum_to_if() might be a possible approach, but strictly speaking it would >>>> still be racy since there is still no lock against device removal. I have not tried >>>> calling usb_lock_device() in usb_set_interface() - would that possibly be an option ? >>> >>> As far as I know, protecting against these races is the responsibility >>> of the USB interface drivers. They must make sure that their disconnect >>> routines block until all outstanding calls to usb_set_interface return >>> (in fact, until all outstanding device accesses have finished). >>> >>> For instance, in the log extract you showed, it's obvious that the >>> uvc_start_streaming routine was running after the disconnect routine had >>> returned, which looks like a bug in itself: Once the disconnect routine >>> returns, the driver is not supposed to try to access the device at all >>> because some other driver may now be bound to it. >>> >>> We can't just call usb_lock_device from within usb_set_interface, >>> because usb_set_interface is often called with that lock already held. >>> >> I had a closer look into the uvcvideo driver and compared it to other usb >> drivers, including drivers in drivers/media/usb/ which connect to the video >> subsystem. >> >> The usbvideo driver lacks protection against calls to uvc_disconnect() while > > Are you confusing usbvideo and uvcvideo ? Both exist, and uvcvideo would > have been called usbvideo if the former hadn't already been in use. > Yes, sorry :-(. I am not sure how s/uvc/usb/ happened. >> calls into file operations are ongoing. This is pretty widespread, and not >> even limited to file operations (for example, there is a worker which is only >> canceled in uvc_delete, not in ucv_disconnect). The existing protection only >> ensures that no file operations are started after the call to ucv_disconnect, >> but that is insufficient. >> >> Other drivers do have that protection and make sure that no usb operations >> can happen after the disconnect call. >> >> The only remedy I can see is to rework the usbvideo driver and add the >> necessary protections. At first glance, it looks like this may be a >> substantial amount of work. I'd sign up for that, but before I start, >> I would like to get input from the usbvideo community. Is such an effort >> already going on ? If yes, how can I help ? If not, is the problem >> understood and accepted ? Are there any ideas on how to solve it ? > > This is something that has been discussed before, and needs to be solved > in the V4L2 framework itself, not in individual drivers. Not only would > this avoid rolling out the same code manually everywhere (in different > incorrect ways, as races are difficult to solve and implementations are > more often wrong than right), but it will also avoid similar issues for > non-USB devices. > You mean code that ensures that no user-space v4l2 operation is in progress after video_device_unregister / v4l2_device_unregister return ? I agree, that would simplify the necessary changes on the uvc side. I actually came from the other side - I assumed that there is a reason that is not already the case, and that the problem therefore has to be resolved on the driver side. So I guess the next question is: Is this already being addressed on the v4l2 side ? > It shouldn't take more than two flags (to track user-space operations in > progress and disconnection), a spinlock and a wait queue entry. I'm not > sure if someone has already given it a try, and don't recall why this > hasn't been done yet, as it should be fairly straightforward. > > On the UVC side, the work queue probably has to be flushed in > uvc_disconnect(). I'd keep the destroy call in uvc_delete() though. > Please make sure to look for potential race conditions between the URB > completion handler and the .disconnect() handler (they shouldn't be any, > but I haven't checked lately myself). > My current solution for this problem is to call uvc_ctrl_cleanup_device() from uvc_disconnect(), after uvc_unregister_video(). An alternative might be to add a uvc_ctrl_stop_device() function which would just cancel the worker. Thanks, Guenter